package issue

import (
	"fmt"
	"regexp"
	"strings"
)

// A checkbox is the one part of a body that is *state* and not prose, so the
// format gives it markup of its own. It is item markup, not a property of one
// section: `## Acceptance criteria` is the usual home, but a type/feature
// keeps its children as checkboxes under `## Issues`. The scan is therefore
// over the whole text and the heading is only recorded, never required.
var (
	// The trailing group stands in for a lookahead RE2 does not have: after
	// the bracket there is either whitespace and then anything, or end of line.
	checkboxRe = regexp.MustCompile(
		`^(?P<indent>[ \t]*)(?P<marker>[-*+]|\d+[.)])[ \t]+` +
			`\[(?P<box>[ xX])\](?P<text>[ \t].*|)$`)
	// Any list item — a sibling ends the item above it, checkbox or not.
	listItemRe = regexp.MustCompile(`^[ \t]*([-*+]|\d+[.)])([ \t]|$)`)
	fenceRe    = regexp.MustCompile("^[ \t]{0,3}(`{3,}|~{3,})")
)

// Checkbox is one checkbox item found in a text.
type Checkbox struct {
	// Index is the 1-based position in the list — what a user types to pick it.
	Index int
	// Line is the 1-based line of the `- [ ]` marker, in the text given.
	Line int
	// EndLine is the 1-based last line of the item, continuations included.
	EndLine int
	// Checked is true for [x] / [X].
	Checked bool
	// Text is the item's text; continuation lines joined with one space.
	Text string
	// Section is the nearest preceding `## ` heading, "" above the first one.
	Section string
}

// Checkboxes returns every checkbox item in text, in document order.
//
// A pure function of the string it is given — no I/O, no store, no tracker.
// Pass an issue body to get body-relative line numbers, or a whole file to get
// file-relative ones; nothing else changes.
//
// Rules:
//
//   - Only a line matching checkboxRe opens an item. A wrapped ("continuation")
//     line is part of the item above it, never an item of its own; the item runs
//     to the next blank line, heading, code fence, or list marker.
//   - Fenced code blocks are skipped whole: `- [ ]` inside a fence is an example
//     of the markup, not a box anybody may tick.
//   - `-`, `*`, `+` and `1.` markers all count, at any indentation, so nested
//     lists are seen too.
func Checkboxes(text string) []Checkbox {
	lines := splitLines(text)
	var items []Checkbox
	section, fence := "", ""

	for n, line := range lines {
		if m := fenceRe.FindStringSubmatch(line); m != nil {
			tok := m[1]
			switch {
			case fence == "":
				fence = tok
			case tok[0] == fence[0] && len(tok) >= len(fence):
				fence = ""
			}
			continue
		}
		if fence != "" {
			continue
		}
		if strings.HasPrefix(line, "## ") {
			section = strings.TrimSpace(line)
			continue
		}
		if strings.HasPrefix(line, "# ") {
			section = ""
			continue
		}
		m := checkboxRe.FindStringSubmatch(line)
		if m == nil {
			continue
		}
		end := n + 1
		parts := []string{strings.TrimSpace(m[4])}
		for k := n + 1; k < len(lines); k++ {
			next := lines[k]
			if strings.TrimSpace(next) == "" || strings.HasPrefix(next, "#") ||
				fenceRe.MatchString(next) || listItemRe.MatchString(next) {
				break
			}
			end = k + 1
			parts = append(parts, strings.TrimSpace(next))
		}
		var kept []string
		for _, p := range parts {
			if p != "" {
				kept = append(kept, p)
			}
		}
		items = append(items, Checkbox{
			Index:   len(items) + 1,
			Line:    n + 1,
			EndLine: end,
			Checked: m[3] != " ",
			Text:    strings.Join(kept, " "),
			Section: section,
		})
	}
	return items
}

// SetCheckbox returns text with the checkbox on the given 1-based line set to
// checked.
//
// Pure, and deliberately surgical: exactly one byte of the input changes — the
// one between the brackets. Everything else, including trailing whitespace and
// the item's own wording, comes back byte for byte. That is the whole point:
// ticking a box must not produce a diff wider than the state that changed.
//
// Already in the requested state is a no-op — text comes back unchanged, and
// an existing [X] keeps its capital.
func SetCheckbox(text string, line int, checked bool) (string, error) {
	off := 0
	for n := 1; off <= len(text); n++ {
		nl := strings.IndexByte(text[off:], '\n')
		var raw string
		if nl == -1 {
			raw = text[off:]
		} else {
			raw = text[off : off+nl]
		}
		if n == line {
			m := checkboxRe.FindStringSubmatchIndex(strings.TrimRight(raw, "\r"))
			if m == nil {
				return "", fmt.Errorf("line %d is not a checkbox item", line)
			}
			box := off + m[6] // group 3: box
			if (text[box] != ' ') == checked {
				return text, nil
			}
			c := byte(' ')
			if checked {
				c = 'x'
			}
			return text[:box] + string(c) + text[box+1:], nil
		}
		if nl == -1 {
			break
		}
		off += nl + 1
	}
	return "", fmt.Errorf("line %d is past the end of the text", line)
}

// CheckboxProgress is (done, total) over every checkbox in text; (0, 0) when
// it has none.
//
// Computed on the fly, on purpose. Progress is not a metadata field: it is the
// body read back, and the body is the only place the state lives.
func CheckboxProgress(text string) (done, total int) {
	items := Checkboxes(text)
	for _, c := range items {
		if c.Checked {
			done++
		}
	}
	return done, len(items)
}

// splitLines is strings.Split minus the phantom final element a trailing
// newline produces, matching Python's str.splitlines().
func splitLines(text string) []string {
	if text == "" {
		return nil
	}
	lines := strings.Split(strings.ReplaceAll(text, "\r\n", "\n"), "\n")
	if last := len(lines) - 1; lines[last] == "" {
		lines = lines[:last]
	}
	return lines
}