{ lib, stdenv, zig, src }:

stdenv.mkDerivation (finalAttrs: {
  pname = "tally";
  # Kept in step with build.zig.zon.
  version = "0.1.0";

  inherit src;

  __structuredAttrs = true;
  strictDeps = true;

  # Zig resolves build.zig.zon dependencies (libvaxis) over the network, which the
  # build sandbox forbids. `fetchDeps` runs `zig build --fetch` inside a fixed-output
  # derivation, so the fetch happens once against a known hash and the real build
  # stays offline.
  #
  # This hash changes whenever build.zig.zon's dependencies change. To update it: set
  # `lib.fakeHash`, run the build, and paste the hash nix reports.
  zigDeps = zig.fetchDeps {
    inherit (finalAttrs) pname version src;
    fetchAll = true;
    hash = "sha256-Wy/H3z2dBsF/WTF2LrCofOuE15pbgdwEJLSUrKCOQGI=";
  };

  postConfigure = ''
    # A writable copy rather than a symlink into the store: Zig writes cache
    # metadata alongside the fetched packages while it verifies them.
    cp -rLT ${finalAttrs.zigDeps} "$ZIG_GLOBAL_CACHE_DIR/p"
    chmod -R u+w "$ZIG_GLOBAL_CACHE_DIR/p"
  '';

  nativeBuildInputs = [ zig ];

  # The zig setup hook's check phase runs `zig build test`, with the same flags as the
  # build (ReleaseSafe): the engine, CLI, TUI, C ABI and JNI tests, and the C program
  # linked through tally.h. None of them touch the network or anything outside the
  # build directory, so every rebuild is a test run. (`jvm-test`, `coverage` and
  # `android` are separate steps, not run here.)
  doCheck = true;

  meta = {
    description = "Calculator with exact arithmetic, unit conversion and a programmer mode, as a CLI and TUI";
    longDescription = ''
      One Zig engine behind a command line, a terminal UI and an Android app:
      exact rational arithmetic where the operations allow it, programmer mode
      (8 to 128-bit integers in every base), unit conversion as part of the
      expression language, and financial functions. This package is the CLI/TUI
      binary, plus the engine as a static library and as a shared library with
      its C header (tally.h).
    '';
    homepage = "https://git.lerch.org/lobo/tally";
    license = lib.licenses.mit;
    mainProgram = "tally";
    platforms = lib.platforms.unix;
  };
})