package main

import (
	"bytes"
	"context"
	"encoding/base64"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"sort"
	"strconv"
	"strings"
	"time"
)

type conflictFileLoader func(
	context.Context, string, int, string, string, string, string,
) ([]string, error)

type conflictFileResult struct {
	files     []string
	err       error
	checkedAt time.Time
}

func (c *GitHubClient) loadConflictFiles(
	ctx context.Context,
	repositoryURL string,
	number int,
	baseRef, baseOID, headOID string,
) ([]string, error) {
	key := strings.Join([]string{repositoryURL, baseOID, headOID}, "\x00")
	c.conflictMu.Lock()
	cached, ok := c.conflictCache[key]
	c.conflictMu.Unlock()
	if ok && (cached.err == nil || time.Since(cached.checkedAt) < time.Minute) {
		return append([]string(nil), cached.files...), cached.err
	}

	files, err := c.conflicts(ctx, repositoryURL, number, baseRef, baseOID, headOID, c.token)
	result := conflictFileResult{
		files: append([]string(nil), files...), err: err, checkedAt: time.Now(),
	}
	c.conflictMu.Lock()
	c.conflictCache[key] = result
	c.conflictMu.Unlock()
	return files, err
}

func analyzeConflictFiles(
	ctx context.Context,
	repositoryURL string,
	number int,
	baseRef, _, _, token string,
) ([]string, error) {
	if repositoryURL == "" || baseRef == "" || number <= 0 {
		return nil, errors.New("missing repository merge metadata")
	}
	gitDir, err := os.MkdirTemp("", "diple-conflicts-*")
	if err != nil {
		return nil, fmt.Errorf("create temporary merge repository: %w", err)
	}
	defer os.RemoveAll(gitDir)

	run := func(args ...string) ([]byte, error) {
		command := exec.CommandContext(ctx, "git", args...)
		command.Env = gitAuthenticationEnvironment(callerEnvironment(), token)
		return command.CombinedOutput()
	}
	if output, runErr := run("init", "--bare", gitDir); runErr != nil {
		return nil, commandError("initialize merge analysis", output, runErr)
	}
	cloneURL := strings.TrimSuffix(strings.TrimSuffix(repositoryURL, "/"), ".git") + ".git"
	if output, runErr := run("-C", gitDir, "remote", "add", "origin", cloneURL); runErr != nil {
		return nil, commandError("configure merge analysis remote", output, runErr)
	}
	if output, runErr := run("-C", gitDir, "config", "remote.origin.promisor", "true"); runErr != nil {
		return nil, commandError("configure partial clone", output, runErr)
	}
	if output, runErr := run("-C", gitDir, "config", "remote.origin.partialclonefilter", "blob:none"); runErr != nil {
		return nil, commandError("configure partial clone filter", output, runErr)
	}

	refspecs := []string{
		"+refs/heads/" + baseRef + ":refs/diple/base",
		"+refs/pull/" + strconv.Itoa(number) + "/head:refs/diple/head",
	}
	fetch := func(depthArgs ...string) error {
		args := []string{"-C", gitDir, "fetch", "--quiet", "--no-tags", "--filter=blob:none"}
		args = append(args, depthArgs...)
		args = append(args, "origin")
		args = append(args, refspecs...)
		output, runErr := run(args...)
		return commandError("fetch merge inputs", output, runErr)
	}
	if err := fetch("--depth=64"); err != nil {
		return nil, err
	}
	for _, deepen := range []string{"192", "768"} {
		if mergeBaseExists(run, gitDir) {
			break
		}
		if err := fetch("--deepen=" + deepen); err != nil {
			return nil, err
		}
	}
	if !mergeBaseExists(run, gitDir) {
		if err := fetch("--unshallow"); err != nil {
			return nil, err
		}
	}

	output, runErr := run(
		"-C", gitDir, "merge-tree", "--write-tree", "--name-only", "--no-messages", "-z",
		"refs/diple/base", "refs/diple/head",
	)
	if runErr == nil {
		return nil, nil
	}
	var exitErr *exec.ExitError
	if !errors.As(runErr, &exitErr) || exitErr.ExitCode() != 1 {
		return nil, commandError("analyze merge conflicts", output, runErr)
	}
	return parseConflictFiles(output)
}

func mergeBaseExists(
	run func(...string) ([]byte, error),
	gitDir string,
) bool {
	_, err := run(
		"-C", gitDir, "merge-base", "refs/diple/base", "refs/diple/head",
	)
	return err == nil
}

func parseConflictFiles(output []byte) ([]string, error) {
	parts := bytes.Split(output, []byte{0})
	if len(parts) < 2 || len(parts[0]) == 0 {
		return nil, errors.New("git merge-tree returned malformed conflict data")
	}
	files := make([]string, 0, len(parts)-2)
	seen := make(map[string]bool)
	for _, raw := range parts[1:] {
		name := string(raw)
		if name == "" || seen[name] {
			continue
		}
		seen[name] = true
		files = append(files, name)
	}
	sort.Strings(files)
	return files, nil
}

func commandError(action string, output []byte, err error) error {
	if err == nil {
		return nil
	}
	message := strings.TrimSpace(string(output))
	if message == "" {
		return fmt.Errorf("%s: %w", action, err)
	}
	return fmt.Errorf("%s: %s", action, message)
}

func callerEnvironment() []string {
	const prefix = "GIT_CONFIG_"
	environment := make([]string, 0, len(os.Environ())+5)
	for _, item := range os.Environ() {
		if !strings.HasPrefix(item, prefix) && !strings.HasPrefix(item, "GIT_TERMINAL_PROMPT=") {
			environment = append(environment, item)
		}
	}
	return environment
}

func gitAuthenticationEnvironment(environment []string, token string) []string {
	environment = append(environment, "GIT_TERMINAL_PROMPT=0")
	if token == "" {
		return environment
	}
	credentials := base64.StdEncoding.EncodeToString([]byte("x-access-token:" + token))
	return append(environment,
		"GIT_CONFIG_COUNT=1",
		"GIT_CONFIG_KEY_0=http.extraHeader",
		"GIT_CONFIG_VALUE_0=Authorization: Basic "+credentials,
	)
}