// Command coverage-summary prints a whole-project coverage summary from a Go
// coverage profile (cover.out).
//
// Batch helper for `make coverage-summary`. After `go test -coverprofile` writes
// a profile, this prints a file-weighted project number and the source files
// present on disk but absent from the profile.
//
// The value here is the missing-file detection: a package no test exercises
// never appears in the profile, so a statement-weighted total silently skips it
// and the suite looks healthier than it is. This counts such a file as 0% and
// weights the project number by file rather than by statement, so untested
// files stay visible. `go tool cover -func` already prints the per-function /
// per-file table, so this deliberately doesn't reimplement that.
//
// Self-contained on the standard library, so it runs anywhere the bundle lands
// via `go run coverage-summary.go`. The cover.out line shape it parses is:
//
//	<import-path>/<file>.go:startLine.col,endLine.col numStmts count
//
// Profile paths are import-path-qualified; the module path from the project's
// go.mod maps them back to on-disk relative paths.
//
// Usage:
//
//	go run coverage-summary.go PROFILE SOURCE_DIR [PROJECT_ROOT]
//	  PROFILE      path to the cover.out coverage profile
//	  SOURCE_DIR   the directory to account for (walked recursively for *.go)
//	  PROJECT_ROOT module root holding go.mod; defaults to the current directory
package main

import (
	"bufio"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
)

type counts struct{ covered, total int }

func die(format string, args ...any) {
	fmt.Fprintf(os.Stderr, "coverage-summary: "+format+"\n", args...)
	os.Exit(1)
}

// readModule returns the module path from PROJECT_ROOT/go.mod, or "" if absent.
func readModule(root string) string {
	f, err := os.Open(filepath.Join(root, "go.mod"))
	if err != nil {
		return ""
	}
	defer f.Close()
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if rest, ok := strings.CutPrefix(line, "module "); ok {
			return strings.TrimSpace(rest)
		}
	}
	_ = sc.Err() // a read error just means an unknown module path; paths stay unstripped
	return ""
}

// stripModule turns an import-path-qualified file into a module-relative path.
func stripModule(path, module string) string {
	if module != "" {
		if rest, ok := strings.CutPrefix(path, module+"/"); ok {
			return rest
		}
	}
	return path
}

// parseProfile reads a cover.out profile into module-relative path -> counts.
// COVERED is the statements in blocks hit at least once; TOTAL is all statements.
func parseProfile(profile, module string) map[string]counts {
	f, err := os.Open(profile)
	if err != nil {
		die("coverage profile not found: %s", profile)
	}
	defer f.Close()
	acc := map[string]counts{}
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		line := strings.TrimSpace(sc.Text())
		if line == "" || strings.HasPrefix(line, "mode:") {
			continue
		}
		// "<path>:<start>.<col>,<end>.<col> <numStmts> <count>"
		fields := strings.Fields(line)
		if len(fields) != 3 {
			continue
		}
		idx := strings.LastIndex(fields[0], ":")
		if idx < 0 {
			continue
		}
		rel := stripModule(fields[0][:idx], module)
		ns, err1 := strconv.Atoi(fields[1])
		cnt, err2 := strconv.Atoi(fields[2])
		if err1 != nil || err2 != nil {
			continue
		}
		c := acc[rel]
		c.total += ns
		if cnt > 0 {
			c.covered += ns
		}
		acc[rel] = c
	}
	if err := sc.Err(); err != nil {
		die("reading %s: %v", profile, err)
	}
	return acc
}

// underDir keeps the entries whose path is at or below sourceRel.
func underDir(table map[string]counts, sourceRel string) map[string]counts {
	if sourceRel == "." || sourceRel == "" {
		return table
	}
	prefix := sourceRel + "/"
	out := map[string]counts{}
	for path, c := range table {
		if path == sourceRel || strings.HasPrefix(path, prefix) {
			out[path] = c
		}
	}
	return out
}

// sourceFiles returns *.go files under sourceDir (relative to root), skipping
// test files, vendored code, and the local tooling footprint.
func sourceFiles(sourceDir, root string) []string {
	var out []string
	_ = filepath.WalkDir(sourceDir, func(path string, d os.DirEntry, err error) error {
		if err != nil {
			return nil
		}
		if d.IsDir() {
			switch d.Name() {
			case "vendor", ".git", ".claude", "testdata":
				return filepath.SkipDir
			}
			return nil
		}
		name := d.Name()
		if !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") {
			return nil
		}
		rel, err := filepath.Rel(root, path)
		if err == nil {
			out = append(out, filepath.ToSlash(rel))
		}
		return nil
	})
	sort.Strings(out)
	return out
}

func missing(tracked map[string]counts, sourceDir, root string) []string {
	var out []string
	for _, f := range sourceFiles(sourceDir, root) {
		if _, ok := tracked[f]; !ok {
			out = append(out, f)
		}
	}
	return out
}

func filePct(c counts) float64 {
	if c.total > 0 {
		return 100.0 * float64(c.covered) / float64(c.total)
	}
	return 100.0
}

func projectPct(tracked map[string]counts, miss []string) float64 {
	total := len(tracked) + len(miss)
	if total == 0 {
		return 0.0
	}
	score := 0.0
	for _, c := range tracked {
		score += filePct(c)
	}
	return score / float64(total)
}

func summaryText(tracked map[string]counts, miss []string, sourceRel string) string {
	total := len(tracked) + len(miss)
	pct := projectPct(tracked, miss)
	var b strings.Builder
	fmt.Fprintf(&b, "Coverage summary for %s\n\n", sourceRel)
	fmt.Fprintf(&b, "Project coverage: %.1f%% (%d tracked, %d missing, %d total; missing files count as 0%%)\n\n",
		pct, len(tracked), len(miss), total)
	plural := "s"
	if len(miss) == 1 {
		plural = ""
	}
	fmt.Fprintf(&b, "Not in coverage report: %d file%s\n", len(miss), plural)
	if len(miss) > 0 {
		b.WriteString("These files had no coverage entry; they count as 0% in project coverage.\n")
		sort.Strings(miss)
		for _, m := range miss {
			fmt.Fprintf(&b, "  %s\n", m)
		}
	} else {
		b.WriteString("Every source file appears in the coverage report.\n")
	}
	b.WriteString("\n(Per-file table: run `go tool cover -func=<profile>`.)\n")
	return b.String()
}

func main() {
	args := os.Args[1:]
	if len(args) < 2 || len(args) > 3 {
		fmt.Fprintln(os.Stderr, "usage: coverage-summary.go PROFILE SOURCE_DIR [PROJECT_ROOT]")
		os.Exit(2)
	}
	profile, sourceDir := args[0], args[1]
	root := "."
	if len(args) == 3 {
		root = args[2]
	}
	absRoot, err := filepath.Abs(root)
	if err != nil {
		die("bad project root %s: %v", root, err)
	}
	absSource, err := filepath.Abs(sourceDir)
	if err != nil {
		die("bad source dir %s: %v", sourceDir, err)
	}
	sourceRel, err := filepath.Rel(absRoot, absSource)
	if err != nil {
		die("source dir %s is not under project root %s", sourceDir, root)
	}
	sourceRel = filepath.ToSlash(sourceRel)

	module := readModule(absRoot)
	tracked := underDir(parseProfile(profile, module), sourceRel)
	miss := missing(tracked, absSource, absRoot)

	fmt.Println()
	fmt.Print(summaryText(tracked, miss, sourceRel))
}