// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package main

import (
	"slices"
	"strings"
	"testing"
)

func lines(ss ...string) []string { return ss }

func TestUnifiedDiffIdentical(t *testing.T) {
	if got := unifiedDiff("f", lines("a", "b"), lines("a", "b")); got != "" {
		t.Errorf("identical inputs produced %q, want empty", got)
	}
}

func TestUnifiedDiffSingleChange(t *testing.T) {
	a := lines("1", "2", "3", "4", "5", "6", "7", "8")
	b := lines("1", "2", "3!", "4", "5", "6", "7", "8")
	want := "--- f\n+++ f\n" +
		"@@ -1,6 +1,6 @@\n" +
		" 1\n 2\n-3\n+3!\n 4\n 5\n 6\n"
	if got := unifiedDiff("f", a, b); got != want {
		t.Errorf("diff = %q, want %q", got, want)
	}
}

func TestUnifiedDiffInsertAtStart(t *testing.T) {
	got := unifiedDiff("f", lines("x"), lines("new", "x"))
	// The single existing line is shown as trailing context, so the hunk
	// covers it.
	want := "--- f\n+++ f\n@@ -1,1 +1,2 @@\n+new\n x\n"
	if got != want {
		t.Errorf("diff = %q, want %q", got, want)
	}
}

func TestUnifiedDiffDeleteAtEnd(t *testing.T) {
	got := unifiedDiff("f", lines("x", "y"), lines("x"))
	want := "--- f\n+++ f\n@@ -1,2 +1,1 @@\n x\n-y\n"
	if got != want {
		t.Errorf("diff = %q, want %q", got, want)
	}
}

func TestUnifiedDiffTwoHunks(t *testing.T) {
	var a, b []string
	for i := 1; i <= 20; i++ {
		a = append(a, itoa(i))
		b = append(b, itoa(i))
	}
	b[1] = "2!"
	b[17] = "18!"
	got := unifiedDiff("f", a, b)
	if !strings.Contains(got, "@@ -1,5 +1,5 @@\n 1\n-2\n+2!\n 3\n 4\n 5\n") {
		t.Errorf("first hunk wrong:\n%s", got)
	}
	if !strings.Contains(got, "@@ -15,6 +15,6 @@\n 15\n 16\n 17\n-18\n+18!\n 19\n 20\n") {
		t.Errorf("second hunk wrong:\n%s", got)
	}
}

// TestUnifiedDiffAdjacentHunks merges changes separated by exactly twice the
// context into one hunk.
func TestUnifiedDiffAdjacentHunks(t *testing.T) {
	a := lines("1", "2", "3", "4", "5", "6", "7", "8")
	b := slices.Clone(a)
	b[0] = "1!"
	b[7] = "8!"
	got := unifiedDiff("f", a, b)
	want := "--- f\n+++ f\n" +
		"@@ -1,8 +1,8 @@\n" +
		"-1\n+1!\n 2\n 3\n 4\n 5\n 6\n 7\n-8\n+8!\n"
	if got != want {
		t.Errorf("diff = %q, want %q", got, want)
	}
}

func itoa(n int) string {
	if n == 0 {
		return "0"
	}
	var buf [4]byte
	i := len(buf)
	for n > 0 {
		i--
		buf[i] = byte('0' + n%10)
		n /= 10
	}
	return string(buf[i:])
}