deb packaging: build-deb.sh + packaging/deb, icons from ico/

- scripts/build-deb.sh: binary built in ubuntu:22.04 (glibc 2.35 baseline),
  Depends via dpkg-shlibdeps, DEB_REVISION (0.1.28-1), changelog.Debian.gz,
  postinst/postrm cache refresh, REPO=1 local apt index, self-checks
  (version/upgrade path, stale binary, desktop-file-validate)
- packaging/deb/: control.in, rcalc.desktop, appdata metainfo, changelog, hooks
- ico/: new app icon in 32/48/64/96/256/512, used for the hicolor slots
- convert.rs: fix clippy::approx_constant error in roundtrip test (PI constant)
- README: install/update/release instructions

.gitignore

@@ -1,8 +1,13 @@
 /target
 /target-appimage
+/target-deb
 /.cache
 /dist/*.AppImage
 /dist/rcalc.AppDir/
+/dist/deb-root/
+/dist/*.deb
+/dist/Packages
+/dist/Packages.gz
 /dist/.glibc-build.txt
 .rcalc_last_bump
 /target-windows

Cargo.lock

@@ -3904,7 +3904,7 @@ dependencies = [
 
 [[package]]
 name = "rcalc"
-version = "0.1.21"
+version = "0.1.28"
 dependencies = [
  "slint",
  "slint-build",

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rcalc"
-version = "0.1.21"
+version = "0.1.28"
 edition = "2021"
 build = "build.rs"
 

README.md

@@ -12,6 +12,70 @@ cargo run --release
 ./dist/rcalc
 ```
 
+## Install as a Debian package
+
+```bash
+./scripts/build-deb.sh                            # → dist/rcalc_<version>-<rev>_<arch>.deb
+sudo apt install ./dist/rcalc_*_amd64.deb
+rcalc                                             # or start it from the application menu
+```
+
+The binary is compiled in an Ubuntu 22.04 container (Docker) by default, so the
+package keeps a glibc 2.35 baseline and installs on Ubuntu 22.04 / 24.04 / 26.04.
+`Depends:` are generated from the ELF with `dpkg-shlibdeps`.
+
+Menu/taskbar icons come from `ico/` — every PNG there is placed into the
+hicolor slot matching its real pixel size (file names do not matter), and the
+remaining smaller sizes are downsampled from the biggest one. Without `ico/`
+the icons are resized from `assets/icon.png`. The icon inside the window
+(`ui/app.slint`) and the Windows `rcalc.exe` icon still use `assets/`.
+
+Useful switches:
+
+| Variable | Effect |
+| --- | --- |
+| `DEB_REVISION=` | packaging revision, `1` by default → `Version: 0.1.28-1` |
+| `REPO=1` | also write `dist/Packages(.gz)` so `dist/` works as a local apt repo |
+| `SKIP_DOCKER=1` | build on the host (needs this host's glibc on the target) |
+| `PREBUILT=1` | package the existing `dist/rcalc`, no compile |
+| `NO_STRIP=1` | keep symbols (bigger package) |
+| `KEEP_STAGE=1` | leave `dist/deb-root/` for inspection |
+| `DOCKER_IMAGE=` | other build image, e.g. `debian:12` for an older baseline |
+| `DEB_MAINTAINER=` | `"Name <mail>"` in `Maintainer:` (default: git user) |
+
+### Version and updates
+
+Package version = `<version from Cargo.toml>-<DEB_REVISION>`, e.g. `0.1.28-1`.
+
+* changed app code → bump `version` in `Cargo.toml` (it is also the window title);
+* changed only packaging (icon, `.desktop`, `Depends`, rebuilt binary) → `DEB_REVISION=2`.
+
+`dpkg` compares these in order (`0.1.28-1 < 0.1.28-2 < 0.1.29-1`, and
+`0.1.9 < 0.1.10`), so every new build is an upgrade, never a downgrade. The
+script prints the resulting upgrade path and warns if the version being built is
+not newer than the installed one — an `apt upgrade` of such a package does nothing.
+Release notes live in `packaging/deb/changelog` and are installed as
+`/usr/share/doc/rcalc/changelog.Debian.gz`.
+
+Update in place (config in `~/.config/rcalc/config.json` is a user file, it survives):
+
+```bash
+sudo apt install ./dist/rcalc_0.1.28-2_amd64.deb
+```
+
+Or keep `dist/` as a local repository and update with plain `apt` (build with
+`REPO=1`, then point apt at the folder):
+
+```bash
+echo 'deb [trusted=yes] file:/path/to/RCalc/dist ./' | sudo tee /etc/apt/sources.list.d/rcalc.list
+sudo apt update && sudo apt install rcalc          # later: sudo apt upgrade
+```
+
+> If `scripts/install-desktop.sh` was ever run, `~/.local/bin/rcalc` (a symlink to
+> `dist/rcalc`) shadows `/usr/bin/rcalc` — remove it after switching to the package,
+> otherwise `rcalc` in the terminal still starts the old binary. The script warns
+> about this during the build.
+
 ### Icon (Linux / Wayland)
 
 On **Wayland** the window `icon` property is ignored — the desktop uses
@@ -27,8 +91,9 @@ clicking a random binary path, so KDE/GNOME can match `app_id=rcalc` to
 
 ## Features
 
-- Modes: **Standard**, **Programmer**, **Convert**
-- Standard: `+ − × ÷`, `%`, `±`, `1/x`, `x²`, `√x`, memory
+- Modes: **Standard**, **Engineering**, **Programmer**, **Convert**, **CRC**, **Settings**
+- Standard: `+ − × ÷`, `%`, `±`, `1/x`, `x²`, `√x`, memory, formula panel
+- Engineering: trig functions, log/pow, DEG/RAD, SI prefixes
 - Programmer: simultaneous **HEX / DEC / OCT / BIN**, sizes **QWORD / DWORD / DINT / WORD / INT / BYTE**,
   bitwise ops, clickable bits
   (WORD/DWORD — unsigned DEC; INT/DINT — signed)
@@ -46,3 +111,7 @@ clicking a random binary path, so KDE/GNOME can match `app_id=rcalc` to
 - `src/main.rs` — UI wiring
 - `ui/app.slint` — Slint interface
 - `dist/rcalc` — release binary
+- `scripts/build-deb.sh` + `packaging/deb/` — .deb package (control, desktop
+  file, AppStream metainfo, maintainer scripts)
+- `scripts/build-appimage.sh` — portable AppImage
+- `scripts/install-desktop.sh` — per-user launcher + icon (no root needed)

build.rs

@@ -1,7 +1,4 @@
 use std::env;
-use std::fs;
-use std::path::PathBuf;
-use std::time::Duration;
 
 fn main() {
     let version = resolve_app_version();
@@ -38,82 +35,10 @@ fn embed_windows_icon() {
 
 /// Window / package version shown as `Rcalc-{version}`.
 ///
-/// On each `release` build the patch in `Cargo.toml` is incremented once
-/// (`0.1.0` → `0.1.1` → …). If Cargo restarts the build script right after we
-/// rewrite `Cargo.toml`, a short mtime guard prevents a double bump.
+/// Always the version from `Cargo.toml`. Раньше каждая release-сборка
+/// инкрементировала patch в Cargo.toml посреди сборки — из-за этого
+/// имя AppImage (читается скриптом до сборки) отставало от версии,
+/// зашитой в бинарник. Теперь версия меняется только вручную.
 fn resolve_app_version() -> String {
-    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
-    let pkg = env::var("CARGO_PKG_VERSION").unwrap_or_else(|_| "0.0.0".into());
-    let profile = env::var("PROFILE").unwrap_or_default();
-
-    if profile != "release" {
-        return pkg;
-    }
-
-    let cargo_toml = manifest_dir.join("Cargo.toml");
-    let guard_path = manifest_dir.join(".rcalc_last_bump");
-    let already = fs::read_to_string(&guard_path)
-        .map(|s| s.trim().to_string())
-        .unwrap_or_default();
-
-    // Cargo re-invoked us because we just rewrote Cargo.toml — keep version.
-    if already == pkg {
-        if let Ok(meta) = fs::metadata(&cargo_toml) {
-            if let Ok(modified) = meta.modified() {
-                if modified.elapsed().unwrap_or(Duration::MAX) < Duration::from_secs(15) {
-                    return pkg;
-                }
-            }
-        }
-    }
-
-    let Some(next) = bump_patch(&pkg) else {
-        return pkg;
-    };
-
-    if let Err(e) = set_cargo_toml_version(&cargo_toml, &next) {
-        println!("cargo:warning=could not bump Cargo.toml version: {e}");
-        return pkg;
-    }
-    let _ = fs::write(&guard_path, &next);
-    next
-}
-
-fn bump_patch(version: &str) -> Option<String> {
-    let mut parts: Vec<&str> = version.split('.').collect();
-    if parts.is_empty() {
-        return None;
-    }
-    while parts.len() < 3 {
-        parts.push("0");
-    }
-    let patch: u64 = parts[2]
-        .split(['-', '+'])
-        .next()?
-        .parse()
-        .ok()?;
-    Some(format!("{}.{}.{}", parts[0], parts[1], patch + 1))
-}
-
-fn set_cargo_toml_version(path: &PathBuf, version: &str) -> std::io::Result<()> {
-    let raw = fs::read_to_string(path)?;
-    let mut out = String::with_capacity(raw.len() + 8);
-    let mut replaced = false;
-    for line in raw.lines() {
-        if !replaced && line.starts_with("version") && line.contains('=') {
-            out.push_str(&format!("version = \"{version}\""));
-            out.push('\n');
-            replaced = true;
-        } else {
-            out.push_str(line);
-            out.push('\n');
-        }
-    }
-    if !replaced {
-        return Err(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "version field not found in Cargo.toml",
-        ));
-    }
-    fs::write(path, out)
+    env::var("CARGO_PKG_VERSION").unwrap_or_else(|_| "0.0.0".into())
 }

packaging/deb/changelog

@@ -0,0 +1,12 @@
+rcalc (0.1.28-1) unstable; urgency=medium
+
+  * Upstream 0.1.28 (version from Cargo.toml), first Debian packaging.
+  * Binary is built in an Ubuntu 22.04 container (glibc 2.35 baseline), so the
+    package installs on Ubuntu 22.04 / 24.04 / 26.04.
+  * Ships /usr/bin/rcalc, /usr/share/applications/rcalc.desktop, hicolor icons
+    taken from ico/, AppStream metainfo and postinst/postrm cache refresh hooks.
+  * Depends are generated from the ELF with dpkg-shlibdeps.
+  * Build fix: clippy::approx_constant error in the convert round-trip test
+    (3.1415927 -> std::f32::consts::PI).
+
+ -- alexMB5300 <prom-ing@yandex.ru>  Thu, 17 Sep 2026 19:12:31 +0300

packaging/deb/control.in

@@ -0,0 +1,23 @@
+Package: rcalc
+Version: @VERSION@
+Architecture: @ARCHITECTURE@
+Maintainer: @MAINTAINER@
+Section: math
+Priority: optional
+Installed-Size: @INSTALLED_SIZE@
+Depends: @DEPENDS@
+Recommends: libegl1, libgl1, libx11-6, libwayland-client0
+Description: desktop calculator with programmer, engineering and convert tools
+ Calculator inspired by the Windows Calculator (standard mode), written in
+ Rust with the Slint UI toolkit. Modes: Standard, Engineering, Programmer,
+ Convert, CRC and Settings.
+ .
+ Programmer mode shows HEX / DEC / OCT / BIN at once, with QWORD / DWORD /
+ DINT / WORD / INT / BYTE word sizes, bitwise operators and clickable bits.
+ .
+ Convert mode maps a float32 to two 16-bit PLC words in every byte order
+ (AB CD / CD AB / BA DC / DC BA), approximates electronic gear ratios as
+ num/den and converts Char / Hex / Dec ASCII codes.
+ .
+ CRC mode computes Modbus RTU CRC-16 and Modbus ASCII LRC from a string of
+ hex bytes, e.g. "01 03 00 00 00 0A".

packaging/deb/postinst

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Maintainer script of the generated rcalc .deb (see scripts/build-deb.sh).
+set -e
+
+case "$1" in
+    configure)
+        ;;
+    abort-upgrade|abort-remove|abort-deconfigure)
+        exit 0
+        ;;
+    *)
+        echo "postinst called with unknown argument \`$1'" >&2
+        exit 1
+        ;;
+esac
+
+# Refresh the XDG caches. Never fail the installation because of them: the
+# tools may be absent (minimal systems, chroot, dpkg -x in a container).
+if command -v update-desktop-database >/dev/null 2>&1; then
+    update-desktop-database -q /usr/share/applications || true
+fi
+
+if command -v gtk-update-icon-cache >/dev/null 2>&1; then
+    gtk-update-icon-cache -q -t -f /usr/share/icons/hicolor || true
+fi
+
+if command -v appstreamcli >/dev/null 2>&1; then
+    appstreamcli refresh --force >/dev/null 2>&1 || true
+fi
+
+exit 0

packaging/deb/postrm

@@ -0,0 +1,27 @@
+#!/bin/sh
+# Maintainer script of the generated rcalc .deb (see scripts/build-deb.sh).
+set -e
+
+case "$1" in
+    remove|upgrade|disappear|failed-upgrade|abort-install|abort-upgrade|purge)
+        ;;
+    *)
+        echo "postrm called with unknown argument \`$1'" >&2
+        exit 1
+        ;;
+esac
+
+# Rebuild the caches after the files were (or are about to be) removed.
+if command -v update-desktop-database >/dev/null 2>&1; then
+    update-desktop-database -q /usr/share/applications || true
+fi
+
+if command -v gtk-update-icon-cache >/dev/null 2>&1; then
+    gtk-update-icon-cache -q -t -f /usr/share/icons/hicolor || true
+fi
+
+if command -v appstreamcli >/dev/null 2>&1; then
+    appstreamcli refresh --force >/dev/null 2>&1 || true
+fi
+
+exit 0

packaging/deb/rcalc.appdata.xml.in

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- @RELEASE_DATE@ = UTC date of the build; @VERSION@ comes from Cargo.toml. -->
+<component type="desktop-application">
+  <id>rcalc.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <!-- LICENSE file is missing in the repository: NOASSERTION until it is added. -->
+  <project_license>NOASSERTION</project_license>
+  <name>Rcalc</name>
+  <summary>Calculator with programmer, engineering, convert and CRC modes</summary>
+  <description>
+    <p>
+      Desktop calculator inspired by the Windows Calculator. Standard mode uses
+      exact rational arithmetic, so 0.1 + 0.2 really gives 0.3.
+    </p>
+    <ul>
+      <li>Standard: + − × ÷, %, ±, 1/x, x², √x, memory, formula panel</li>
+      <li>Engineering: trigonometry, log/pow, DEG/RAD, SI prefixes</li>
+      <li>Programmer: simultaneous HEX / DEC / OCT / BIN, QWORD … BYTE word
+        sizes, bitwise ops, clickable bits</li>
+      <li>Convert: float32 to two 16-bit words (AB CD / CD AB / BA DC / DC BA),
+        gear ratio num/den, ASCII codes</li>
+      <li>CRC: Modbus RTU CRC-16 and Modbus ASCII LRC from hex bytes</li>
+      <li>Full keyboard input, three themes, three button sizes</li>
+    </ul>
+  </description>
+  <categories>
+    <category>Utility</category>
+    <category>Calculator</category>
+    <category>Science</category>
+  </categories>
+  <releases>
+    <release version="@VERSION@" date="@RELEASE_DATE@"/>
+  </releases>
+  <content_rating type="oars-1.1"/>
+</component>

packaging/deb/rcalc.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=Rcalc
+GenericName=Calculator
+Comment=Calculator with Programmer, Engineering, Convert and CRC tools
+Exec=rcalc
+Icon=rcalc
+Terminal=false
+Categories=Utility;Calculator;
+Keywords=calculator;programmer;hex;bitwise;modbus;crc;lrc;
+StartupNotify=true
+# Wayland app_id / X11 WM_CLASS — must match set_xdg_app_id("rcalc") in src/main.rs
+StartupWMClass=rcalc

scripts/build-appimage.sh

@@ -101,6 +101,9 @@ package_appimage() {
   echo "Bundling libraries with linuxdeploy…"
   export APPIMAGE_EXTRACT_AND_RUN=1
   export OUTPUT="${IMAGE_NAME}"
+  # Marker: mtime boundary to find the FRESH linuxdeploy output.
+  local marker="${OUT_DIR}/.pack-start"
+  touch "${marker}"
   (
     cd "${OUT_DIR}"
     "${linuxdeploy}" \
@@ -111,11 +114,15 @@ package_appimage() {
       --output appimage
   )
 
-  # linuxdeploy drops the AppImage next to AppDir (dist/)
+  # linuxdeploy drops the AppImage next to AppDir (dist/). Only consider files
+  # NEWER than the marker: a blind `find | head -1` with several older
+  # *.AppImage in dist/ renamed an OLD image OVER the freshly built one
+  # (rcalc-0.1.28 got shipped with a 0.1.25 binary from the first build).
   local built
-  built="$(find "${OUT_DIR}" -maxdepth 1 -name '*.AppImage' ! -name 'linuxdeploy*' | head -1)"
+  built="$(find "${OUT_DIR}" -maxdepth 1 -name '*.AppImage' ! -name 'linuxdeploy*' -newer "${marker}" | head -1)"
+  rm -f "${marker}"
   if [[ -z "$built" ]]; then
-    echo "error: AppImage was not produced" >&2
+    echo "error: AppImage was not produced (linuxdeploy failed?)" >&2
     exit 1
   fi
   if [[ "$(basename "$built")" != "${IMAGE_NAME}" ]]; then

scripts/build-deb.sh

@@ -0,0 +1,439 @@
+#!/usr/bin/env bash
+# Build a Debian package (.deb) for rcalc.
+#
+# By default the release binary is compiled inside Ubuntu 22.04 (glibc 2.35) —
+# the same baseline and the same cargo cache (target-appimage) as
+# scripts/build-appimage.sh — so the package installs on Ubuntu 22.04 / 24.04
+# and not only on this rolling host distro.
+#
+# Package layout (FHS):
+#   /usr/bin/rcalc                                  stripped release binary
+#   /usr/share/applications/rcalc.desktop           launcher (Exec=rcalc)
+#   /usr/share/icons/hicolor/*x*/apps/rcalc.png     icons from ico/ (or assets/)
+#   /usr/share/metainfo/rcalc.appdata.xml           AppStream metadata
+#   /usr/share/doc/rcalc/README.md
+#
+# Env switches:
+#   DEB_REVISION     packaging revision appended to the Cargo.toml version
+#                    (0.1.28 + 1 → "Version: 0.1.28-1"); bump it for updates that
+#                    change only the package, not the app
+#   REPO=1           also generate dist/Packages(.gz) → dist/ usable as an apt repo
+#   SKIP_DOCKER=1    build on the host instead of Docker (fast, but the deb will
+#                    then require this host's glibc — see DEPENDS below)
+#   PREBUILT=1       do not compile, package the existing dist/rcalc as-is
+#   NO_STRIP=1       keep symbols in the binary (much bigger package)
+#   KEEP_STAGE=1     leave dist/deb-root/ around for inspection
+#   DOCKER_IMAGE     build container image (default: ubuntu:22.04)
+#   DEB_MAINTAINER   "Name <mail>" (default: git user.name/user.email)
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+PKG="rcalc"
+OUT_DIR="${OUT_DIR:-${ROOT}/dist}"
+META_DIR="${ROOT}/packaging/deb"
+STAGE="${OUT_DIR}/deb-root"
+ARCH="$(dpkg --print-architecture 2>/dev/null || uname -m)"
+VERSION="$(grep -m1 '^version' "${ROOT}/Cargo.toml" | sed 's/.*"\(.*\)".*/\1/')"
+# Debian version = upstream (Cargo.toml) + packaging revision: "0.1.28-1".
+# The revision lets you ship an update (fixed desktop file, new icon, rebuilt
+# binary) WITHOUT touching Cargo.toml; dpkg compares 0.1.28-1 < 0.1.28-2 <
+# 0.1.29-1 correctly. DEB_REVISION is the knob for that.
+DEB_REVISION="${DEB_REVISION:-1}"
+DEB_VERSION="${VERSION}-${DEB_REVISION}"
+DEB_FILE="${OUT_DIR}/${PKG}_${DEB_VERSION}_${ARCH}.deb"
+
+DOCKER_IMAGE="${DOCKER_IMAGE:-ubuntu:22.04}"
+SKIP_DOCKER="${SKIP_DOCKER:-0}"
+PREBUILT="${PREBUILT:-0}"
+NO_STRIP="${NO_STRIP:-0}"
+KEEP_STAGE="${KEEP_STAGE:-0}"
+# Nested $( ) with quotes inside ${:- } is parsed unreliably by bash — resolve first.
+GIT_NAME="$(git -C "${ROOT}" config user.name 2>/dev/null || true)"
+GIT_MAIL="$(git -C "${ROOT}" config user.email 2>/dev/null || true)"
+MAINTAINER="${DEB_MAINTAINER:-${GIT_NAME:-${PKG}-packaging} <${GIT_MAIL:-localhost}>}"
+
+need_cmd() {
+  command -v "$1" >/dev/null 2>&1 || {
+    echo "error: required command not found: $1" >&2
+    exit 1
+  }
+}
+
+# ---------------------------------------------------------------- binary ----
+
+build_binary_docker() {
+  need_cmd docker
+  echo "Building release binary in ${DOCKER_IMAGE} (glibc 2.35 baseline)…"
+  mkdir -p "${HOME}/.cargo/registry" "${HOME}/.cargo/git"
+  docker run --rm \
+    -v "${ROOT}:/src" \
+    -v "${HOME}/.cargo/registry:/root/.cargo/registry" \
+    -v "${HOME}/.cargo/git:/root/.cargo/git" \
+    -w /src \
+    "${DOCKER_IMAGE}" \
+    bash -lc '
+      set -euo pipefail
+      export DEBIAN_FRONTEND=noninteractive
+      apt-get update -qq
+      apt-get install -y -qq curl build-essential pkg-config libfontconfig1-dev ca-certificates
+      curl --proto "=https" --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal
+      # shellcheck disable=SC1091
+      source "$HOME/.cargo/env"
+      export CARGO_TARGET_DIR=/src/target-appimage
+      cargo build --release
+      install -D -m755 /src/target-appimage/release/rcalc /src/dist/rcalc
+      ldd --version | head -1 > /src/dist/.glibc-build.txt
+      # Make artifacts writable by the host user
+      chown -R "$(id -u):$(id -g)" /src/dist/rcalc /src/dist/.glibc-build.txt /src/target-appimage || true
+    '
+}
+
+build_binary_host() {
+  echo "Building release binary on the host ($(ldd --version | head -1))…"
+  (cd "${ROOT}" && cargo build --release)
+  install -D -m755 "${ROOT}/target/release/${PKG}" "${OUT_DIR}/${PKG}"
+  ldd --version | head -1 > "${OUT_DIR}/.glibc-build.txt" || true
+}
+
+ensure_binary() {
+  mkdir -p "${OUT_DIR}"
+  if [[ "${PREBUILT}" == "1" ]]; then
+    echo "PREBUILT=1: packaging ${OUT_DIR}/${PKG} as-is"
+  elif [[ "${SKIP_DOCKER}" == "1" ]]; then
+    build_binary_host
+  else
+    build_binary_docker
+  fi
+
+  if [[ ! -x "${OUT_DIR}/${PKG}" ]]; then
+    echo "error: ${OUT_DIR}/${PKG} missing — run without PREBUILT=1" >&2
+    exit 1
+  fi
+}
+
+# ---------------------------------------------------------------- staging ---
+
+stage_tree() {
+  rm -rf "${STAGE}"
+  mkdir -p \
+    "${STAGE}/DEBIAN" \
+    "${STAGE}/usr/bin" \
+    "${STAGE}/usr/share/applications" \
+    "${STAGE}/usr/share/metainfo" \
+    "${STAGE}/usr/share/doc/${PKG}"
+
+  install -m755 "${OUT_DIR}/${PKG}" "${STAGE}/usr/bin/${PKG}"
+  if [[ "${NO_STRIP}" != "1" ]] && command -v strip >/dev/null 2>&1; then
+    strip --strip-unneeded "${STAGE}/usr/bin/${PKG}"
+  fi
+
+  install -m644 "${META_DIR}/${PKG}.desktop" \
+    "${STAGE}/usr/share/applications/${PKG}.desktop"
+
+  sed -e "s|@VERSION@|${VERSION}|" \
+      -e "s|@RELEASE_DATE@|$(date -u +%Y-%m-%d)|" \
+      "${META_DIR}/${PKG}.appdata.xml.in" \
+      > "${STAGE}/usr/share/metainfo/${PKG}.appdata.xml"
+
+  gzip -9 -c "${ROOT}/README.md" > "${STAGE}/usr/share/doc/${PKG}/README.md.gz"
+  echo "Binary: $(file -b "${STAGE}/usr/bin/${PKG}" | cut -d, -f1-3)"
+}
+
+# Icons for the desktop entry: /usr/share/icons/hicolor/<N>x<N>/apps/rcalc.png
+#
+# Source of truth, in order:
+#   1. ico/*.png — ready-made sizes dropped in by hand. The REAL pixel size of
+#      each file decides which hicolor slot it fills (file names are ignored),
+#      and ico/ wins for the whole family: mixing ico/ artwork with assets/
+#      artwork would show a different picture depending on the icon size.
+#   2. assets/icon.png resized to the standard sizes (needs ImageMagick).
+#   3. the three PNGs bundled in assets/, as-is.
+stage_icons() {
+  local dst="${STAGE}/usr/share/icons/hicolor"
+  local prepared=0 master="" master_size=0 size
+
+  local magick=""
+  for tool in magick convert; do
+    if command -v "${tool}" >/dev/null 2>&1; then
+      magick="${tool}"
+      break
+    fi
+  done
+
+  if [[ -d "${ROOT}/ico" ]] && [[ -n "${magick}" ]]; then
+    for f in "${ROOT}"/ico/*.png; do
+      [[ -f "${f}" ]] || continue
+      local w h
+      read -r w h <<<"$(identify -format '%w %h' "${f}" 2>/dev/null || echo '0 0')" || true
+      [[ "${w}" == "${h}" ]] && [[ "${w}" -gt 0 ]] || continue
+      if (( w > master_size )); then
+        master="${f}"
+        master_size="${w}"
+      fi
+      local slot="${dst}/${w}x${w}/apps/${PKG}.png"
+      if [[ ! -e "${slot}" ]]; then
+        mkdir -p "${dst}/${w}x${w}/apps"
+        install -m644 "${f}" "${slot}"
+        prepared=$((prepared + 1))
+      fi
+    done
+  fi
+
+  if (( prepared > 0 )); then
+    # Fill the remaining standard sizes from the biggest ico/ file — never
+    # upscale, so a 512px source does not produce a blurry 1024px icon.
+    if [[ -n "${magick}" ]]; then
+      for size in 16 24 32 48 64 96 128 256 512; do
+        [[ -e "${dst}/${size}x${size}/apps/${PKG}.png" ]] && continue
+        (( size <= master_size )) || continue
+        mkdir -p "${dst}/${size}x${size}/apps"
+        "${magick}" "${master}" -resize "${size}x${size}" \
+          "${dst}/${size}x${size}/apps/${PKG}.png"
+      done
+    fi
+    echo "Icons:         ${prepared} ready-made from ico/ (master ${master_size}px)"
+    return 0
+  fi
+
+  if [[ -n "${magick}" ]]; then
+    for size in 16 24 32 48 64 128 256 512 1024; do
+      mkdir -p "${dst}/${size}x${size}/apps"
+      "${magick}" "${ROOT}/assets/icon.png" -resize "${size}x${size}" \
+        "${dst}/${size}x${size}/apps/${PKG}.png"
+    done
+    echo "Icons:         resized from assets/icon.png (1024px master)"
+  else
+    install -D -m644 "${ROOT}/assets/icon-64.png" "${dst}/64x64/apps/${PKG}.png"
+    install -D -m644 "${ROOT}/assets/icon-256.png" "${dst}/256x256/apps/${PKG}.png"
+    install -D -m644 "${ROOT}/assets/icon.png" "${dst}/1024x1024/apps/${PKG}.png"
+    echo "Icons:         assets/*.png as-is (ImageMagick not found)"
+  fi
+}
+
+# ------------------------------------------------------------ dependencies --
+
+# Real library dependencies, read from the ELF itself. GL / X11 / Wayland
+# libraries are dlopen()ed by winit at runtime, so they never show up in
+# NEEDED — they are listed in Recommends in control.in instead.
+compute_depends() {
+  local depends=""
+  if [[ "${USE_SHLIBDEPS:-1}" == "1" ]] && command -v dpkg-shlibdeps >/dev/null 2>&1; then
+    local tmp
+    tmp="$(mktemp -d)"
+    mkdir -p "${tmp}/debian"
+    # dpkg-shlibdeps insists on reading a debian/control next to it.
+    cat > "${tmp}/debian/control" <<EOF
+Source: ${PKG}
+
+Package: ${PKG}
+Architecture: ${ARCH}
+Depends: \${shlibs:Depends}
+Description: temporary, for dpkg-shlibdeps
+EOF
+    : > "${tmp}/debian/${PKG}.substvars"
+    (cd "${tmp}" && dpkg-shlibdeps -T"debian/${PKG}.substvars" "${STAGE}/usr/bin/${PKG}") \
+      >/dev/null 2>&1 || true
+    depends="$(sed -n 's/^shlibs:Depends=//p' "${tmp}/debian/${PKG}.substvars")"
+    rm -rf "${tmp}"
+  fi
+
+  if [[ -z "${depends}" ]]; then
+    echo "warning: dpkg-shlibdeps gave nothing, using a hand-written Depends list" >&2
+    depends='libc6, libfontconfig1, libgcc-s1'
+  fi
+  printf '%s' "${depends}"
+}
+
+# ---------------------------------------------------------------- control ---
+
+stage_control() {
+  local installed_size depends
+  installed_size="$(du -s -k --exclude="${STAGE}/DEBIAN" "${STAGE}" | awk '{print $1}')"
+  depends="$(compute_depends)"
+
+  sed -e "s|@VERSION@|${DEB_VERSION}|" \
+      -e "s|@ARCHITECTURE@|${ARCH}|" \
+      -e "s|@MAINTAINER@|${MAINTAINER}|" \
+      -e "s|@INSTALLED_SIZE@|${installed_size}|" \
+      -e "s|@DEPENDS@|${depends}|" \
+      "${META_DIR}/control.in" > "${STAGE}/DEBIAN/control"
+
+  # Maintainer scripts must be executables owned by root in the archive.
+  install -m755 "${META_DIR}/postinst" "${STAGE}/DEBIAN/postinst"
+  install -m755 "${META_DIR}/postrm" "${STAGE}/DEBIAN/postrm"
+
+  (cd "${STAGE}" && find . -path ./DEBIAN -prune -o -type f -print \
+    | sed 's|^\./||' | sort | xargs -r md5sum) > "${STAGE}/DEBIAN/md5sums"
+
+  echo "Version:        ${DEB_VERSION}  (upstream ${VERSION} + revision ${DEB_REVISION})"
+  echo "Architecture:   ${ARCH}"
+  echo "Maintainer:     ${MAINTAINER}"
+  echo "Installed-Size: ${installed_size} KB"
+  echo "Depends:        ${depends}"
+}
+
+
+# ------------------------------------------------------- version / update ---
+
+# What "apt upgrade" needs: a version that is strictly greater than the one
+# already installed, and a package name that does not change. Both are checked
+# here so a release is never silently unpromotable.
+check_version() {
+  case "${VERSION}" in
+    [0-9]*) ;;
+    *) echo "error: upstream version '${VERSION}' must start with a digit (dpkg rule)" >&2; exit 1 ;;
+  esac
+  if [[ ! "${VERSION}" =~ ^[0-9A-Za-z.+-~:]+$ || ! "${DEB_REVISION}" =~ ^[0-9A-Za-z.+-~]+$ ]]; then
+    echo "error: bad version '${DEB_VERSION}' — allowed chars are letters, digits and . + - ~ :" >&2
+    exit 1
+  fi
+
+  local installed
+  installed="$(dpkg-query -W -f='${Version}' "${PKG}" 2>/dev/null || true)"
+  if [[ -n "${installed}" ]]; then
+    if dpkg --compare-versions "${installed}" ge "${DEB_VERSION}"; then
+      echo "warning: ${PKG} ${installed} is ALREADY installed and is not older than ${DEB_VERSION}" >&2
+      echo "         → 'apt upgrade' will do nothing. Bump Cargo.toml or DEB_REVISION." >&2
+    else
+      echo "Upgrade path:   ${installed} → ${DEB_VERSION}  (apt upgrade will pick this up)"
+    fi
+  else
+    echo "Upgrade path:   fresh install of ${DEB_VERSION}"
+  fi
+
+  # scripts/install-desktop.sh links ~/.local/bin/rcalc → dist/rcalc, and
+  # ~/.local/bin comes before /usr/bin in PATH: the packaged binary would be
+  # shadowed by an old copy even after a successful upgrade.
+  if [[ -e "${HOME}/.local/bin/${PKG}" ]]; then
+    echo "warning: ${HOME}/.local/bin/${PKG} exists and shadows /usr/bin/${PKG}" >&2
+    echo "         (installed by scripts/install-desktop.sh — remove it after switching to the .deb)" >&2
+  fi
+}
+
+# /usr/share/doc/rcalc/changelog.Debian.gz — Debian policy expects a changelog,
+# and it is the human-readable history of what every version changed.
+# packaging/deb/changelog (kept in git) wins; otherwise one entry is generated
+# from the current commit so the package is never changelog-less.
+stage_changelog() {
+  local doc="${STAGE}/usr/share/doc/${PKG}"
+  mkdir -p "${doc}"
+
+  local plain="${doc}/changelog.Debian"     # gzip → changelog.Debian.gz (Debian-side history)
+  if [[ -f "${META_DIR}/changelog" ]]; then
+    cp "${META_DIR}/changelog" "${plain}"
+    echo "Changelog:      packaging/deb/changelog"
+  else
+    local githash gitsubject
+    githash="$(git -C "${ROOT}" rev-parse --short HEAD 2>/dev/null || echo unknown)"
+    gitsubject="$(git -C "${ROOT}" log -1 --pretty=%s 2>/dev/null || echo packaging)"
+    {
+      printf '%s (%s) unstable; urgency=medium\n\n' "${PKG}" "${DEB_VERSION}"
+      printf '  * Upstream %s, packaged as %s.\n' "${VERSION}" "${DEB_VERSION}"
+      if [[ -f "${OUT_DIR}/.glibc-build.txt" ]]; then
+        printf '  * Built against %s\n' "$(cat "${OUT_DIR}/.glibc-build.txt")"
+      fi
+      printf '  * git %s: %s\n\n' "${githash}" "${gitsubject}"
+      printf ' -- %s  %s\n' "${MAINTAINER}" "$(date -R)"
+    } > "${plain}"
+    echo "Changelog:      generated (add packaging/deb/changelog to keep real history)"
+  fi
+
+  gzip -9n "${plain}"                       # -n: no timestamp → reproducible .gz
+  rm -f "${plain}"
+}
+
+# REPO=1 → turn dist/ into a flat apt repository, so updates can be installed
+# with plain `apt update && apt upgrade` instead of pointing at a .deb file.
+make_repo() {
+  [[ "${REPO:-0}" == "1" ]] || return 0
+  need_cmd dpkg-scanpackages
+  (
+    cd "${OUT_DIR}"
+    # One pass: Packages is the index, Packages.gz the compressed copy apt reads.
+    # Keep both files: apt reads Packages.gz, humans diff/read Packages.
+    dpkg-scanpackages --multiversion . /dev/null > Packages 2>/dev/null
+    gzip -9n -c Packages > Packages.gz
+  )
+  echo "Repo index:     ${OUT_DIR}/Packages.gz ($(grep -c '^Package:' "${OUT_DIR}/Packages") entries)"
+  echo "                apt one-liner:  echo 'deb [trusted=yes] file:${OUT_DIR} ./' \\"
+  echo "                                 | sudo tee /etc/apt/sources.list.d/${PKG}.list"
+  echo "                then:           sudo apt update && sudo apt install ${PKG}"
+}
+
+# ------------------------------------------------------------------ build ---
+
+build_package() {
+  need_cmd dpkg-deb
+  rm -f "${DEB_FILE}"
+
+  # The stage tree inherits the build user's umask (often 002 → group-writable).
+  # Debian expects 755 dirs / 644 data files, plus the exec bits on the binary
+  # and the maintainer scripts.
+  find "${STAGE}" -type d -exec chmod 0755 {} +
+  find "${STAGE}" -type f -exec chmod 0644 {} +
+  chmod 0755 "${STAGE}/usr/bin/${PKG}" \
+             "${STAGE}/DEBIAN/postinst" "${STAGE}/DEBIAN/postrm"
+
+  # --root-owner-group: the stage tree is owned by the build user, the archive
+  # must not be (dpkg rejects foreign uids unless forced).
+  dpkg-deb --root-owner-group --build "${STAGE}" "${DEB_FILE}"
+}
+
+verify_package() {
+  local ok=1
+
+  # Fields straight from the archive (dpkg-deb --info output is localised).
+  dpkg-deb --field "${DEB_FILE}"
+
+  if command -v desktop-file-validate >/dev/null 2>&1; then
+    if desktop-file-validate "${STAGE}/usr/share/applications/${PKG}.desktop"; then
+      echo "desktop-file-validate: OK"
+    else
+      echo "warning: the desktop file did not validate" >&2
+      ok=0
+    fi
+  fi
+
+  # dist/rcalc can be stale: build.rs embeds APP_VERSION from Cargo.toml only at
+  # compile time, and the AppImage packaging had exactly that bug (old binary in
+  # a new image). Anything newer than the packaged binary means "rebuild it".
+  local stale
+  stale="$(find "${ROOT}/src" "${ROOT}/ui" "${ROOT}/build.rs" "${ROOT}/Cargo.toml" \
+    -newer "${OUT_DIR}/${PKG}" -print -quit 2>/dev/null || true)"
+  if [[ -n "${stale}" ]]; then
+    echo "warning: ${stale} is newer than the packaged binary" >&2
+    echo "         (PREBUILT=1 / SKIP_DOCKER=1 reused an old dist/${PKG} — rebuild to be safe)" >&2
+    ok=0
+  fi
+
+  echo "Files in package: $(dpkg-deb -c "${DEB_FILE}" | grep -c '^-')"
+
+  if [[ "${KEEP_STAGE}" != "1" ]]; then
+    rm -rf "${STAGE}"
+  fi
+
+  echo
+  ls -l "${DEB_FILE}"
+  [[ "${ok}" == "1" ]] || echo "(check the warnings above)"
+}
+
+check_version
+ensure_binary
+stage_tree
+stage_icons
+stage_changelog
+stage_control
+build_package
+make_repo
+verify_package
+
+echo
+echo "OK: ${DEB_FILE}"
+echo "Version:  ${DEB_VERSION}  (upstream ${VERSION} + revision ${DEB_REVISION})"
+echo "Install:  sudo apt install '${DEB_FILE}'"
+echo "Next release:  bump version in Cargo.toml (app changes) — or just"
+echo "               DEB_REVISION=2 ./scripts/build-deb.sh (packaging/icon only),"
+echo "               then install the new .deb: apt replaces the old one in place."
+echo "Run:      rcalc   (or start it from the application menu)"
+echo "Remove:   sudo apt remove ${PKG}"
+

src/convert.rs

@@ -391,7 +391,9 @@ impl Converter {
                 if !ch.is_ascii_hexdigit() {
                     return;
                 }
-                if self.draft == "0" {
+                // A synced all-zero draft ("0", "0000") is a placeholder:
+                // first digit replaces it instead of being swallowed.
+                if self.draft.chars().all(|c| c == '0') {
                     self.draft = ch.to_string();
                 } else if self.draft.len() < 4 {
                     self.draft.push(ch);
@@ -478,6 +480,11 @@ impl Converter {
                 if t.is_empty() || t == "-" || t == "." || t == "-." {
                     return Ok(());
                 }
+                // Exponent being typed: `1E`, `1E+`, `1E-` — intermediate
+                // state, keep the previous value without flagging an error.
+                if t.ends_with('E') || t.ends_with("E+") || t.ends_with("E-") {
+                    return Ok(());
+                }
                 match t.parse::<f32>() {
                     Ok(v) => {
                         self.float_bits = v.to_bits();
@@ -777,8 +784,20 @@ fn format_float(v: f32) -> String {
             "Inf".into()
         };
     }
-    let s = format!("{v}");
-    s
+    if v == 0.0 {
+        // Keep IEEE −0 visible (bits 0x8000_0000).
+        return if v.is_sign_negative() {
+            "-0".into()
+        } else {
+            "0".into()
+        };
+    }
+    // Scientific notation for extremes: keeps f32::MAX readable and the
+    // draft re-parsable (`3.4028235e38`.parse::<f32>() works).
+    if v.abs() < 1e-10 || v.abs() >= 1e16 {
+        return format!("{v:e}");
+    }
+    format!("{v}")
 }
 
 /// IEEE float bytes in natural order A B C D (big-endian bit pattern),
@@ -839,7 +858,7 @@ mod tests {
 
     #[test]
     fn roundtrip_all_endians() {
-        let samples = [0.0f32, -1.0, 3.1415927, 12345.678, f32::MIN_POSITIVE];
+        let samples = [0.0f32, -1.0, std::f32::consts::PI, 12345.678, f32::MIN_POSITIVE];
         for e in Endian::all() {
             for &f in &samples {
                 let (w0, w1) = float_to_words(f, e);
@@ -889,4 +908,71 @@ mod tests {
         assert_eq!(c.ascii_code(), b'A');
         assert_eq!(c.ascii_char_text(), "A");
     }
+
+    #[test]
+    fn float_draft_exponent_in_progress() {
+        // Typing `1E` is an intermediate state, not an error.
+        let mut c = Converter::new();
+        c.clear_active(); // draft "0"
+        c.input_char('1');
+        c.input_char('E');
+        assert_eq!(c.error(), None, "typing `1E` must not flag invalid float");
+        c.input_char('5');
+        assert_eq!(c.error(), None);
+        assert_eq!(c.float_value(), 1e5);
+    }
+
+    #[test]
+    fn float_draft_exponent_sign_in_progress() {
+        let mut c = Converter::new();
+        c.clear_active(); // draft "0"
+        c.input_char('1');
+        c.input_char('E');
+        c.input_char('-');
+        assert_eq!(c.error(), None, "typing `1E-` must not flag invalid float");
+        c.input_char('9');
+        assert_eq!(c.error(), None);
+        assert_eq!(c.float_value(), 1e-9);
+    }
+
+    #[test]
+    fn negative_zero_words() {
+        // IEEE -0.0 must round-trip through the word view (0x8000 0000).
+        let mut c = Converter::new();
+        c.clear_active(); // draft "0", float +0
+        c.input_char('-');
+        c.input_char('0');
+        assert_eq!(c.words(), (0x8000, 0x0000));
+        assert_eq!(c.float_text(), "-0");
+    }
+
+    #[test]
+    fn word_edit_updates_float() {
+        let mut c = Converter::new();
+        c.clear_active(); // float = 0
+        c.set_active(ConvField::Word0);
+        for ch in "3F80".chars() {
+            c.input_char(ch);
+        }
+        assert_eq!(c.float_value(), 1.0);
+    }
+
+    #[test]
+    fn huge_float_uses_scientific() {
+        // f32::MAX plain-decimal display is 39 chars; exponent form is
+        // readable and re-parsable by the float draft parser.
+        let t = format_float(f32::MAX);
+        assert!(t.contains('e'), "expected scientific notation, got {t}");
+        assert_eq!(t.parse::<f32>().unwrap(), f32::MAX);
+
+        let t = format_float(1e-20);
+        assert!(t.contains('e'), "expected scientific notation, got {t}");
+        assert_eq!(t.parse::<f32>().unwrap(), 1e-20);
+    }
+
+    #[test]
+    fn negative_zero_displayed() {
+        assert_eq!(format_float(-0.0), "-0");
+        assert_eq!(format_float(0.0), "0");
+    }
 }

src/engine.rs

@@ -77,18 +77,27 @@ impl Rational {
         Self { num: n, den: 1 }
     }
 
-    /// Parse a decimal literal like `12`, `-3.5`, `0.001` into an exact fraction.
+    /// Parse a decimal literal like `12`, `-3.5`, `0.001`, `1.5e-9` into an
+    /// exact fraction (exponent shifts by exact powers of ten).
     fn parse(s: &str) -> Result<Self, CalcError> {
         let s = s.trim();
         if s.is_empty() || s == "." || s == "-" || s == "-." {
             return Err(CalcError::Invalid);
         }
-        let neg = s.starts_with('-');
-        let body = if neg { &s[1..] } else { s };
+        // Split off an optional exponent: `1e-15`, `2.5E+3`.
+        let (body, exp) = match s.split_once(['e', 'E']) {
+            Some((b, e)) => {
+                let exp: i32 = e.parse().map_err(|_| CalcError::Invalid)?;
+                (b, exp)
+            }
+            None => (s, 0i32),
+        };
+        let neg = body.starts_with('-');
+        let b = if neg { &body[1..] } else { body };
 
-        let (int_part, frac_part) = match body.split_once('.') {
-            Some((a, b)) => (a, b),
-            None => (body, ""),
+        let (int_part, frac_part) = match b.split_once('.') {
+            Some((a, c)) => (a, c),
+            None => (b, ""),
         };
         if int_part.is_empty() && frac_part.is_empty() {
             return Err(CalcError::Invalid);
@@ -107,6 +116,27 @@ impl Rational {
             .and_then(|v| v.checked_add(frac_val))
             .ok_or(CalcError::Overflow)?;
         let num = if neg { -num } else { num };
+
+        // Apply the exponent with exact power-of-ten arithmetic:
+        // value·10^exp = num·10^exp / 10^frac_digits.
+        let (num, den) = if exp >= 0 {
+            let e = exp as usize;
+            if e >= frac_digits {
+                (
+                    num.checked_mul(pow10(e - frac_digits)?)
+                        .ok_or(CalcError::Overflow)?,
+                    1,
+                )
+            } else {
+                (num, den / pow10(e)?)
+            }
+        } else {
+            (
+                num,
+                den.checked_mul(pow10((-exp) as usize)?)
+                    .ok_or(CalcError::Overflow)?,
+            )
+        };
         Self::new(num, den)
     }
 
@@ -175,9 +205,14 @@ impl Value {
         if !v.is_finite() {
             return Err(CalcError::Overflow);
         }
-        // Prefer exact integer when float is (almost) integral.
-        if v.fract().abs() < 1e-12 && v.abs() < 1e15 {
-            return Ok(Value::Rat(Rational::from_i128(v.round() as i128)));
+        if v == 0.0 {
+            return Ok(Value::Rat(Rational::from_i128(0)));
+        }
+        // Prefer exact integer when float is (almost) integral. Never snap a
+        // sub-unit magnitude to an integer: that turned 1e-16 into `0`.
+        let r = v.round();
+        if r != 0.0 && (v - r).abs() < 1e-12 && v.abs() < 1e15 {
+            return Ok(Value::Rat(Rational::from_i128(r as i128)));
         }
         Ok(Value::Float(v))
     }
@@ -394,7 +429,7 @@ impl Calculator {
             self.entry = d.to_string();
         } else if self.entry == "-0" {
             self.entry = format!("-{d}");
-        } else if digit_count(&self.entry) < 16 {
+        } else if digit_count(&self.entry) < 40 && significant_digits(&self.entry) < 16 {
             self.entry.push(d);
         }
         self.entry_value = None;
@@ -911,6 +946,19 @@ fn tokenize(src: &str) -> Result<Vec<(Tok, Option<Value>)>, CalcError> {
                 while i < chars.len() && (chars[i].is_ascii_digit() || chars[i] == '.') {
                     i += 1;
                 }
+                // Optional decimal exponent: `1e-9`, `2.5E+3`.
+                if i < chars.len() && (chars[i] == 'e' || chars[i] == 'E') {
+                    let mut j = i + 1;
+                    if j < chars.len() && (chars[j] == '+' || chars[j] == '-') {
+                        j += 1;
+                    }
+                    if j < chars.len() && chars[j].is_ascii_digit() {
+                        i = j;
+                        while i < chars.len() && chars[i].is_ascii_digit() {
+                            i += 1;
+                        }
+                    }
+                }
                 let lit: String = chars[start..i].iter().collect();
                 let v = Value::from_entry(&lit)?;
                 out.push((Tok::Num, Some(v)));
@@ -984,6 +1032,15 @@ fn digit_count(s: &str) -> usize {
     s.chars().filter(|c| c.is_ascii_digit()).count()
 }
 
+/// Significant digits (leading zeros don't count) — caps precision, not
+/// magnitude, so `0.0000000000000001` (1e-16) can be typed.
+fn significant_digits(s: &str) -> usize {
+    s.chars()
+        .filter(|c| c.is_ascii_digit())
+        .skip_while(|&c| c == '0')
+        .count()
+}
+
 fn gcd(mut a: u128, mut b: u128) -> u128 {
     while b != 0 {
         let t = b;
@@ -1016,36 +1073,36 @@ fn format_rational(r: Rational) -> String {
 }
 
 fn terminating_decimal(r: Rational) -> Option<String> {
+    // Terminating iff den = 2^a · 5^b. Digit count after the point is
+    // k = max(a, b); scale num so the denominator becomes exactly 10^k.
     let mut den = r.den;
+    let (mut a, mut b) = (0u32, 0u32);
     while den % 2 == 0 {
         den /= 2;
+        a += 1;
     }
     while den % 5 == 0 {
         den /= 5;
+        b += 1;
     }
     if den != 1 {
         return None;
     }
-    // Scale denominator to a power of 10.
-    let mut d = r.den;
-    let mut n = r.num;
-    let neg = n < 0;
-    n = n.abs();
-    while d % 10 != 0 {
-        if d % 2 == 0 {
-            d *= 5;
-            n *= 5;
-        } else if d % 5 == 0 {
-            d *= 2;
-            n *= 2;
-        } else {
-            break;
-        }
+    let k = a.max(b);
+    let mut n = r.num.abs();
+    for _ in 0..(k - a) {
+        n = n.checked_mul(2)?;
+    }
+    for _ in 0..(k - b) {
+        n = n.checked_mul(5)?;
+    }
+    let mut d: i128 = 1;
+    for _ in 0..k {
+        d = d.checked_mul(10)?;
     }
     let int_part = n / d;
     let mut frac = (n % d).to_string();
-    let width = d.to_string().len().saturating_sub(1);
-    while frac.len() < width {
+    while frac.len() < k as usize {
         frac.insert(0, '0');
     }
     // trim trailing zeros
@@ -1057,24 +1114,26 @@ fn terminating_decimal(r: Rational) -> Option<String> {
     } else {
         format!("{int_part}.{frac}")
     };
-    Some(if neg { format!("-{body}") } else { body })
+    Some(if r.num < 0 { format!("-{body}") } else { body })
 }
 
 fn format_float(v: f64) -> String {
-    if !v.is_finite() {
+    if !v.is_finite() || v == 0.0 {
         return "0".into();
     }
-    // Snap near-integers (guards any float path residue).
-    if (v - v.round()).abs() < 1e-10 && v.abs() < 1e15 {
-        return format!("{}", v.round() as i64);
+    // Snap near-integers (guards any float path residue); never for |v| < 1,
+    // otherwise tiny results like 1e-16 collapse to `0`.
+    let r = v.round();
+    if r != 0.0 && (v - r).abs() < 1e-10 && v.abs() < 1e15 {
+        return format!("{}", r as i64);
     }
-    let s = format!("{:.12}", v);
-    let s = s.trim_end_matches('0').trim_end_matches('.').to_string();
-    if s.is_empty() || s == "-" {
-        "0".into()
-    } else {
-        s
+    // Very small / large magnitudes: scientific notation keeps the value
+    // visible and re-parseable (`Rational::parse` understands exponents).
+    if v.abs() < 1e-10 || v.abs() >= 1e16 {
+        return format!("{v:e}");
     }
+    // Shortest representation that round-trips back to `v` exactly.
+    format!("{v}")
 }
 
 impl fmt::Display for Rational {
@@ -1173,4 +1232,107 @@ mod tests {
         let r = Rational::parse("1.25").unwrap();
         assert_eq!(r, Rational::new(5, 4).unwrap());
     }
+
+    // --- regression tests (rounding / formatting bugs) ---
+
+    #[test]
+    fn quarter_and_eighth_display() {
+        // terminating_decimal scaled the fraction wrong: 1/4 displayed as "0.5".
+        let mut c = Calculator::new();
+        c.input_digit('1');
+        c.set_op(Op::Div);
+        c.input_digit('4');
+        c.equals();
+        assert_eq!(c.display(), "0.25");
+
+        c.clear_all();
+        c.input_digit('1');
+        c.set_op(Op::Div);
+        c.input_digit('8');
+        c.equals();
+        assert_eq!(c.display(), "0.125");
+    }
+
+    #[test]
+    fn five_hundredths_display() {
+        // 0.05 must survive exact formatting (was displayed as "0.1").
+        let mut c = Calculator::new();
+        c.input_digit('0');
+        c.input_dot();
+        c.input_digit('0');
+        c.input_digit('5'); // 0.05
+        c.set_op(Op::Mul);
+        c.input_digit('1');
+        c.equals();
+        assert_eq!(c.display(), "0.05");
+    }
+
+    #[test]
+    fn tiny_reciprocal_not_zero() {
+        // 1/9999999999999999 ≈ 1e-16 was snapped to "0" by format_float.
+        let mut c = Calculator::new();
+        for d in "9999999999999999".chars() {
+            c.input_digit(d);
+        }
+        c.reciprocal();
+        let disp = c.display();
+        assert!(disp != "0", "tiny reciprocal must not display as 0, got {disp}");
+    }
+
+    #[test]
+    fn tiny_result_reusable_standard() {
+        // Float result in scientific notation must be re-parsable as operand.
+        let mut c = Calculator::new();
+        for d in "9999999999999999".chars() {
+            c.input_digit(d);
+        }
+        c.reciprocal();
+        c.set_op(Op::Mul);
+        c.input_digit('2');
+        c.equals();
+        let disp = c.display();
+        assert!(disp != "0" && disp != "Invalid input", "got {disp}");
+    }
+
+    #[test]
+    fn sqrt_square_roundtrip_tiny() {
+        // √(1e-30) = 1e-15 was snapped to 0 by Value::from_f64.
+        let mut c = Calculator::new();
+        c.input_dot(); // "0."
+        for _ in 0..14 {
+            c.input_digit('0');
+        }
+        c.input_digit('1'); // 0.000000000000001 = 1e-15
+        c.square();
+        c.sqrt();
+        let disp = c.display();
+        assert!(disp != "0", "sqrt(1e-30) must not display as 0, got {disp}");
+    }
+
+    #[test]
+    fn rational_parse_exponent() {
+        // Scientific-notation entries must parse to exact fractions.
+        let r = Rational::parse("1e-15").unwrap();
+        assert_eq!(r, Rational::new(1, 1_000_000_000_000_000).unwrap());
+        let r = Rational::parse("1.5e3").unwrap();
+        assert_eq!(r, Rational::new(1500, 1).unwrap());
+        let r = Rational::parse("2.5e-2").unwrap();
+        assert_eq!(r, Rational::new(1, 40).unwrap());
+        let r = Rational::parse("-1E+3").unwrap();
+        assert_eq!(r, Rational::new(-1000, 1).unwrap());
+        assert!(Rational::parse("1e2e3").is_err());
+        assert!(Rational::parse("1e").is_err());
+    }
+
+    #[test]
+    fn can_type_one_e_minus_16() {
+        // Leading zeros must not eat the 16-significant-digit budget.
+        let mut c = Calculator::new();
+        c.input_dot();
+        for _ in 0..15 {
+            c.input_digit('0');
+        }
+        c.input_digit('1');
+        assert_eq!(c.display(), "0.0000000000000001");
+    }
 }

src/engineering.rs

@@ -199,7 +199,7 @@ impl Engineering {
             self.entry = d.to_string();
         } else if self.entry == "-0" {
             self.entry = format!("-{d}");
-        } else if digit_count(&self.entry) < 16 {
+        } else if digit_count(&self.entry) < 40 && significant_digits(&self.entry) < 16 {
             self.entry.push(d);
         }
         self.sync_expr();
@@ -389,6 +389,26 @@ impl Engineering {
         self.sync_expr();
     }
 
+    /// Insert `× 10 ^` for scientific-notation entry: `1.5 [×10ˣ] 3 = 1500`.
+    /// Exponent may be negated with ± before `=` (`10 ^ −3` parses correctly).
+    pub fn mul_pow10(&mut self) {
+        if self.error.is_some() {
+            return;
+        }
+        if self.done {
+            // Continue from the result: 1500 [×10ˣ] 2 = 150000.
+            self.formula.clear();
+            self.done = false;
+            self.typing = true;
+        }
+        self.flush_entry();
+        self.formula.push_str(" × 10 ^ ");
+        self.entry = "0".into();
+        self.typing = false;
+        self.second = false;
+        self.sync_expr();
+    }
+
     pub fn equals(&mut self) {
         if self.error.is_some() {
             return;
@@ -550,20 +570,31 @@ fn digit_count(s: &str) -> usize {
     s.chars().filter(|c| c.is_ascii_digit()).count()
 }
 
+/// Significant digits (leading zeros don't count) — caps precision, not
+/// magnitude, so `0.0000000000000001` (1e-16) can be typed.
+fn significant_digits(s: &str) -> usize {
+    s.chars()
+        .filter(|c| c.is_ascii_digit())
+        .skip_while(|&c| c == '0')
+        .count()
+}
+
 fn format_num(v: f64) -> String {
-    if !v.is_finite() {
+    if !v.is_finite() || v == 0.0 {
         return "0".into();
     }
-    if (v - v.round()).abs() < 1e-10 && v.abs() < 1e15 {
-        return format!("{}", v.round() as i64);
+    // Snap near-integers; never for |v| < 1 (kept 10^−50 displayed as "0").
+    let r = v.round();
+    if r != 0.0 && (v - r).abs() < 1e-10 && v.abs() < 1e15 {
+        return format!("{}", r as i64);
     }
-    let s = format!("{:.12}", v);
-    let s = s.trim_end_matches('0').trim_end_matches('.').to_string();
-    if s.is_empty() || s == "-" {
-        "0".into()
-    } else {
-        s
+    // Scientific notation for extremes; `entry.parse::<f64>()` and the
+    // tokenizer both accept it, so the result stays reusable.
+    if v.abs() < 1e-10 || v.abs() >= 1e16 {
+        return format!("{v:e}");
     }
+    // Shortest round-trip representation.
+    format!("{v}")
 }
 
 // --- expression evaluator ---
@@ -639,6 +670,20 @@ fn tokenize(src: &str) -> Result<Vec<Tok>, SciError> {
                 while i < chars.len() && (chars[i].is_ascii_digit() || chars[i] == '.') {
                     i += 1;
                 }
+                // Optional decimal exponent: `1e-9`, `2.5E+3`. The standalone
+                // constant `e` stays an Ident (needs no digit right after).
+                if i < chars.len() && (chars[i] == 'e' || chars[i] == 'E') {
+                    let mut j = i + 1;
+                    if j < chars.len() && (chars[j] == '+' || chars[j] == '-') {
+                        j += 1;
+                    }
+                    if j < chars.len() && chars[j].is_ascii_digit() {
+                        i = j;
+                        while i < chars.len() && chars[i].is_ascii_digit() {
+                            i += 1;
+                        }
+                    }
+                }
                 let lit: String = chars[start..i].iter().collect();
                 let v: f64 = lit.parse().map_err(|_| SciError::Invalid)?;
                 out.push(Tok::Num(v));
@@ -858,4 +903,91 @@ mod tests {
         e.equals();
         assert_eq!(e.display(), "1024");
     }
+
+    #[test]
+    fn mul_pow10_basic() {
+        // 1.5 ×10ˣ 3 = 1500 (× 10 ^ с приоритетом ^ над ×).
+        let mut e = Engineering::new();
+        e.input_digit('1');
+        e.input_dot();
+        e.input_digit('5');
+        e.mul_pow10();
+        e.input_digit('3');
+        e.equals();
+        assert_eq!(e.display(), "1500");
+    }
+
+    #[test]
+    fn mul_pow10_negative_exponent() {
+        // 2.5 ×10ˣ ±2 = 0.025.
+        let mut e = Engineering::new();
+        e.input_digit('2');
+        e.input_dot();
+        e.input_digit('5');
+        e.mul_pow10();
+        e.input_digit('2');
+        e.negate();
+        e.equals();
+        assert_eq!(e.display(), "0.025");
+    }
+
+    #[test]
+    fn mul_pow10_continues_result() {
+        // 2 × 3 = 6, затем 6 ×10ˣ 2 = 600.
+        let mut e = Engineering::new();
+        e.input_digit('2');
+        e.set_op("×");
+        e.input_digit('3');
+        e.equals();
+        e.mul_pow10();
+        e.input_digit('2');
+        e.equals();
+        assert_eq!(e.display(), "600");
+    }
+
+    #[test]
+    fn ten_pow_negative_not_zero() {
+        // 10 ^ −50 ≈ 1e-50 was snapped to "0" by format_num.
+        let mut e = Engineering::new();
+        e.input_digit('1');
+        e.input_digit('0');
+        e.set_op("^");
+        e.input_digit('5');
+        e.input_digit('0');
+        e.negate();
+        e.equals();
+        let disp = e.display();
+        assert!(disp != "0", "10^−50 must not display as 0, got {disp}");
+    }
+
+    #[test]
+    fn tiny_float_result_continues_expression() {
+        // Result in scientific notation must be re-parsable by the tokenizer.
+        let mut e = Engineering::new();
+        e.input_digit('1');
+        e.input_digit('0');
+        e.set_op("^");
+        e.input_digit('1');
+        e.input_digit('6');
+        e.negate();
+        e.equals();
+        assert_eq!(e.display(), "1e-16");
+        e.set_op("+");
+        e.input_digit('1');
+        e.equals();
+        assert_eq!(e.display(), "1");
+    }
+
+    #[test]
+    fn can_type_one_e_minus_16() {
+        // Leading zeros must not eat the 16-significant-digit budget.
+        let mut e = Engineering::new();
+        e.input_digit('0');
+        e.input_dot();
+        for _ in 0..15 {
+            e.input_digit('0');
+        }
+        e.input_digit('1');
+        assert_eq!(e.display(), "0.0000000000000001");
+    }
 }

src/main.rs

@@ -532,6 +532,7 @@ fn handle_engineering(calc: &mut Engineering, id: &str) {
         "eng:fact" => calc.insert_func("fact"),
         "eng:exp" => calc.insert_func("exp"),
         "eng:tenpow" => calc.insert_func("tenpow"),
+        "eng:mul10pow" => calc.mul_pow10(),
         "eng:log" => calc.insert_func("log"),
         "eng:ln" => calc.insert_func("ln"),
         "MC" => calc.memory_clear(),

src/programmer.rs

@@ -699,4 +699,35 @@ mod tests {
         assert_eq!(p.word_size(), WordSize::Dint);
         assert_eq!(p.dec_text().replace(' ', ""), "-1");
     }
+
+    #[test]
+    fn shift_count_mods_word_size() {
+        // Locks hardware-style semantics: shift count is taken modulo the
+        // word size (1 << 100000000 → 1e8 % 64 == 0 → no shift).
+        let mut p = Programmer::new();
+        p.input_digit('1');
+        p.set_op(ProgOp::Lsh);
+        for ch in "100000000".chars() {
+            p.input_digit(ch);
+        }
+        p.equals();
+        assert_eq!(p.current_bits(), 1);
+    }
+
+    #[test]
+    fn shift_out_of_word_gives_zero() {
+        // 0xFFFF_FFFF_FFFF_FFFF Lsh 63 → only bit 0 survives at bit 63.
+        let mut p = Programmer::new();
+        p.set_base(Base::Hex);
+        for ch in "FFFFFFFFFFFFFFFF".chars() {
+            p.input_digit(ch);
+        }
+        p.set_op(ProgOp::Lsh);
+        p.set_base(Base::Dec); // type the shift count in decimal
+        p.input_digit('6');
+        p.input_digit('3');
+        p.equals();
+        // (u64::MAX << 63) & u64::MAX = 0x8000_0000_0000_0000
+        assert_eq!(p.current_bits(), 0x8000_0000_0000_0000);
+    }
 }

ui/app.slint

@@ -1211,7 +1211,7 @@ export component AppWindow inherits Window {
                     label: root.eng-second ? "³√x" : "√x";
                     clicked => { root.press(root.eng-second ? "eng:cbrt" : "eng:sqrt"); }
                 }
-                FuncBtn { min-h: UiStyle.btn-h-sm; label: "÷"; clicked => { root.press("/"); } }
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "x*10^y"; clicked => { root.press("eng:mul10pow"); } }
             }
             Row {
                 FuncBtn {
@@ -1222,14 +1222,14 @@ export component AppWindow inherits Window {
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "1/x"; clicked => { root.press("eng:inv"); } }
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "|x|"; clicked => { root.press("eng:abs"); } }
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "n!"; clicked => { root.press("eng:fact"); } }
-                FuncBtn { min-h: UiStyle.btn-h-sm; label: "×"; clicked => { root.press("*"); } }
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "÷"; clicked => { root.press("/"); } }
             }
             Row {
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "x^y"; clicked => { root.press("^"); } }
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "("; clicked => { root.press("("); } }
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: ")"; clicked => { root.press(")"); } }
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "CE"; clicked => { root.press("CE"); } }
-                FuncBtn { min-h: UiStyle.btn-h-sm; label: "−"; clicked => { root.press("-"); } }
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "×"; clicked => { root.press("*"); } }
             }
             Row {
                 FuncBtn {
@@ -1240,28 +1240,28 @@ export component AppWindow inherits Window {
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "7"; clicked => { root.press("7"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "8"; clicked => { root.press("8"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "9"; clicked => { root.press("9"); } }
-                FuncBtn { min-h: UiStyle.btn-h-sm; label: "+"; clicked => { root.press("+"); } }
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "−"; clicked => { root.press("-"); } }
             }
             Row {
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "log"; clicked => { root.press("eng:log"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "4"; clicked => { root.press("4"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "5"; clicked => { root.press("5"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "6"; clicked => { root.press("6"); } }
-                DigitBtn { min-h: UiStyle.btn-h-sm; label: "±"; clicked => { root.press("neg"); } }
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "+"; clicked => { root.press("+"); } }
             }
             Row {
                 FuncBtn { min-h: UiStyle.btn-h-sm; label: "ln"; clicked => { root.press("eng:ln"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "1"; clicked => { root.press("1"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "2"; clicked => { root.press("2"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "3"; clicked => { root.press("3"); } }
-                EqBtn { min-h: UiStyle.btn-h-sm; clicked => { root.press("="); } }
+                DigitBtn { min-h: UiStyle.btn-h-sm; label: "±"; clicked => { root.press("neg"); } }
             }
             Row {
+                FuncBtn { min-h: UiStyle.btn-h-sm; label: "exp"; clicked => { root.press("eng:exp"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "0"; clicked => { root.press("0"); } }
                 DigitBtn { min-h: UiStyle.btn-h-sm; label: "."; clicked => { root.press("."); } }
-                FuncBtn { min-h: UiStyle.btn-h-sm; label: "exp"; clicked => { root.press("eng:exp"); } }
-                Rectangle { min-height: 32px; background: transparent; }
                 Rectangle { min-height: 32px; background: transparent; }
+                EqBtn { min-h: UiStyle.btn-h-sm; clicked => { root.press("="); } }
             }
         }
 
@@ -1293,10 +1293,17 @@ export component AppWindow inherits Window {
         if root.mode == 0: GridLayout {
             spacing: 6px;
             Row {
-                if root.std-panel == 0: FuncBtn { label: "%"; clicked => { root.press("%"); } }
-                if root.std-panel == 1: FuncBtn { label: "("; clicked => { root.press("("); } }
-                if root.std-panel == 0: FuncBtn { label: "CE"; clicked => { root.press("CE"); } }
-                if root.std-panel == 1: FuncBtn { label: ")"; clicked => { root.press(")"); } }
+                // `if` внутри Row у GridLayout ломает раскладку Slint
+                // (RepeatedItemTree::grid_layout_input_data) — кнопки теряются.
+                // Поэтому одна кнопка с тернарником вместо пары условных.
+                FuncBtn {
+                    label: root.std-panel == 0 ? "%" : "(";
+                    clicked => { root.press(root.std-panel == 0 ? "%" : "("); }
+                }
+                FuncBtn {
+                    label: root.std-panel == 0 ? "CE" : ")";
+                    clicked => { root.press(root.std-panel == 0 ? "CE" : ")"); }
+                }
                 FuncBtn { label: "C"; clicked => { root.press("C"); } }
                 FuncBtn { label: "⌫"; clicked => { root.press("BS"); } }
             }