chore: add code coverage tooling (kcov + Codecov)

.gitignore

@@ -1,5 +1,6 @@
 .zig-cache/
 zig-out/
+coverage/
 
 # Override global gitignore — vendored SQLite is intentional
 !vendor/

README.md

@@ -5,6 +5,7 @@
 ![macOS only](https://img.shields.io/badge/platform-macOS-blue)
 ![Zig 0.15.x](https://img.shields.io/badge/zig-0.15.x-orange)
 ![License](https://img.shields.io/badge/license-MIT-green)
+![Coverage](.github/badges/coverage.svg)
 [![Built with Devbox](https://www.jetify.com/img/devbox/shield_galaxy.svg)](https://www.jetify.com/devbox/docs/contributor-quickstart/)
 
 malt is a macOS-only package manager written in Zig that consumes Homebrew's existing formula, bottle, cask, and tap ecosystem. It ships as a single binary (`malt`, ~3 MB) with sub-millisecond cold start. malt downloads pre-built bottles from the Homebrew infrastructure — it is a fast client for Homebrew's package registry, not a fork. Requires macOS 11 (Big Sur) or later on Apple Silicon or Intel.

build.zig

@@ -67,6 +67,7 @@ pub fn build(b: *std.Build) void {
     };
 
     const test_step = b.step("test", "Run all unit tests");
+    const test_bin_step = b.step("test-bin", "Install test binaries for coverage (kcov)");
 
     // Shared library module — single root that re-exports all source modules.
     // This avoids "file exists in multiple modules" errors from Zig's module system.
@@ -89,7 +90,12 @@ pub fn build(b: *std.Build) void {
     malt_lib.addOptions("version_string", version_options);
 
     inline for (test_modules) |test_file| {
+        // e.g. "tests/formula_test.zig" → "formula_test" (so each test binary
+        // has a unique install name, which `test-bin` / kcov need).
+        const test_name = comptime std.fs.path.stem(std.fs.path.basename(test_file));
+
         const t = b.addTest(.{
+            .name = test_name,
             .root_module = b.createModule(.{
                 .root_source_file = b.path(test_file),
                 .target = target,
@@ -105,6 +111,13 @@ pub fn build(b: *std.Build) void {
 
         const run_t = b.addRunArtifact(t);
         test_step.dependOn(&run_t.step);
+
+        // Only installed when the user asks for `zig build test-bin`
+        // (used by the coverage recipe and the coverage CI job).
+        const install_t = b.addInstallArtifact(t, .{
+            .dest_dir = .{ .override = .{ .custom = "test-bin" } },
+        });
+        test_bin_step.dependOn(&install_t.step);
     }
 
     // --- Universal binary step (macOS only) ---

justfile

@@ -22,6 +22,74 @@ install:
 test:
     zig build test
 
+# Run tests under kcov, print line-coverage percentage, and refresh
+# the README badge SVG at .github/badges/coverage.svg.
+# HTML report lands at coverage/merged/kcov-merged/index.html.
+# Requires kcov (brew install kcov). Internet is needed for the badge fetch.
+coverage:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    if ! command -v kcov >/dev/null 2>&1; then
+        echo "error: kcov not found. Install with: brew install kcov" >&2
+        exit 1
+    fi
+    rm -rf coverage
+    mkdir -p coverage
+    zig build test-bin
+    # Only report coverage for files under the project's src/ directory.
+    # --include-path takes an absolute path and is more reliable than --include-pattern.
+    src_dir="$(pwd)/src"
+    # Run kcov once per test binary into a shared outdir
+    for bin in zig-out/test-bin/*; do
+        # Skip .dSYM debug bundles and any non-regular files
+        [ -f "$bin" ] || continue
+        [ -x "$bin" ] || continue
+        echo "→ kcov: $(basename "$bin")"
+        kcov --include-path="$src_dir" coverage "$bin" >/dev/null
+    done
+    # kcov 43 on macOS doesn't reliably auto-merge, so do it explicitly.
+    # The per-binary reports are in hash-suffixed dirs (e.g. cellar_test.a934ecd0).
+    shopt -s nullglob
+    per_bin_dirs=(coverage/*_test.*)
+    shopt -u nullglob
+    if [ ${#per_bin_dirs[@]} -eq 0 ]; then
+        echo "error: kcov produced no per-binary reports" >&2
+        exit 1
+    fi
+    kcov --merge coverage/merged "${per_bin_dirs[@]}" >/dev/null
+    report="coverage/merged/kcov-merged/coverage.json"
+    if [ ! -f "$report" ]; then
+        echo "error: merged report not found at $report" >&2
+        exit 1
+    fi
+    if command -v jq >/dev/null 2>&1; then
+        percent=$(jq -r '.percent_covered' "$report")
+    else
+        percent=$(grep -oE '"percent_covered"[^,}]*' "$report" | grep -oE '[0-9]+(\.[0-9]+)?' | head -1)
+    fi
+    # Pick a shields.io color based on the integer part of the percentage.
+    pct_int=${percent%.*}
+    if   [ "$pct_int" -ge 90 ]; then color="brightgreen"
+    elif [ "$pct_int" -ge 80 ]; then color="green"
+    elif [ "$pct_int" -ge 70 ]; then color="yellowgreen"
+    elif [ "$pct_int" -ge 60 ]; then color="yellow"
+    elif [ "$pct_int" -ge 50 ]; then color="orange"
+    else                             color="red"
+    fi
+    # Fetch the static badge SVG from shields.io and commit it under .github/badges/.
+    # This keeps the README badge in-repo so it updates with normal commits — no CI needed.
+    mkdir -p .github/badges
+    badge_url="https://img.shields.io/badge/coverage-${percent}%25-${color}"
+    if curl -sSLf "$badge_url" -o .github/badges/coverage.svg; then
+        badge_msg=".github/badges/coverage.svg (refreshed — remember to commit it)"
+    else
+        badge_msg="warning: could not fetch badge from shields.io (offline?) — badge not refreshed"
+    fi
+    echo ""
+    echo "Coverage: ${percent}%"
+    echo "Report:   coverage/merged/kcov-merged/index.html"
+    echo "Badge:    ${badge_msg}"
+
 # Check formatting
 fmt-check:
     zig fmt --check src/ tests/
@@ -39,6 +107,7 @@ lint:
 # Pre-commit hook: auto-format staged .zig files in place and re-stage them
 # so the formatted version is what actually lands in the commit.
 # Note: if you `git add -p` partial hunks of a file, this will pick up the
+
 # unstaged hunks too — a known limitation of format-on-commit hooks.
 pre-commit:
     #!/usr/bin/env bash