package main

import (
	"encoding/json"
	"os"
	"strings"
)

// diffLines is a line diff of two texts in the style of diff -u without the
// hunk headers: " " unchanged lines near a change, "-" removed, "+" added.
// It returns "" when the texts are equal.
func diffLines(a, b string) string {
	x, y := strings.Split(a, "\n"), strings.Split(b, "\n")
	// Longest common subsequence table.
	lcs := make([][]int, len(x)+1)
	for i := range lcs {
		lcs[i] = make([]int, len(y)+1)
	}
	for i := len(x) - 1; i >= 0; i-- {
		for j := len(y) - 1; j >= 0; j-- {
			if x[i] == y[j] {
				lcs[i][j] = lcs[i+1][j+1] + 1
			} else {
				lcs[i][j] = max(lcs[i+1][j], lcs[i][j+1])
			}
		}
	}
	type line struct {
		op   byte
		text string
	}
	var all []line
	i, j := 0, 0
	for i < len(x) || j < len(y) {
		switch {
		case i < len(x) && j < len(y) && x[i] == y[j]:
			all = append(all, line{' ', x[i]})
			i++
			j++
		case i < len(x) && (j == len(y) || lcs[i+1][j] >= lcs[i][j+1]):
			all = append(all, line{'-', x[i]})
			i++
		default:
			all = append(all, line{'+', y[j]})
			j++
		}
	}
	// Keep two lines of context around changes.
	const ctx = 2
	keep := make([]bool, len(all))
	changed := false
	for k, l := range all {
		if l.op != ' ' {
			changed = true
			for d := -ctx; d <= ctx; d++ {
				if k+d >= 0 && k+d < len(all) {
					keep[k+d] = true
				}
			}
		}
	}
	if !changed {
		return ""
	}
	var out strings.Builder
	skipped := false
	for k, l := range all {
		if !keep[k] {
			skipped = true
			continue
		}
		if skipped {
			out.WriteString("…\n")
			skipped = false
		}
		out.WriteString(string(l.op) + " " + l.text + "\n")
	}
	return out.String()
}

// pretty re-indents JSON with sorted keys, so two spellings of the same
// settings compare equal.
func pretty(data []byte) string {
	var v any
	if json.Unmarshal(data, &v) != nil {
		return strings.TrimSpace(string(data))
	}
	out, _ := json.MarshalIndent(v, "", "  ")
	return string(out)
}

// pendingChanges is the diff from the confirmed settings to the saved ones.
func (s System) pendingChanges() string {
	cur, err1 := os.ReadFile(s.settingsPath())
	conf, err2 := os.ReadFile(s.confirmedPath())
	if err1 != nil || err2 != nil {
		return ""
	}
	return diffLines(pretty(conf), pretty(cur))
}