package docker

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"

	"github.com/moby/patternmatcher"
	"github.com/moby/patternmatcher/ignorefile"
)

// readDockerignore returns the patterns of the files to leave out of the
// build context in contextDir, read as docker build reads them for the
// Dockerfile at dockerfile, a path inside contextDir: from
// <dockerfile>.dockerignore next to the Dockerfile if there is one, otherwise
// from .dockerignore at the root of the context. Without either file there
// are no patterns.
func readDockerignore(contextDir, dockerfile string) ([]string, error) {
	if dockerfile == "" {
		dockerfile = "Dockerfile" // what Docker builds when none is named
	}

	name := dockerfile + ".dockerignore"

	file, err := os.Open(filepath.Join(contextDir, name))
	if errors.Is(err, fs.ErrNotExist) {
		name = ".dockerignore"
		file, err = os.Open(filepath.Join(contextDir, name))
	}

	if errors.Is(err, fs.ErrNotExist) {
		return nil, nil
	}

	if err != nil {
		return nil, fmt.Errorf("failed to read %s: %w", name, err)
	}

	defer func() { _ = file.Close() }()

	excludes, err := ignorefile.ReadAll(file)
	if err != nil {
		return nil, fmt.Errorf("failed to read %s: %w", name, err)
	}

	// Like the docker command line, never leave out .dockerignore or the
	// Dockerfile: Docker reads the Dockerfile from the context.
	for _, keep := range []string{".dockerignore", filepath.ToSlash(dockerfile)} {
		excluded, err := patternmatcher.MatchesOrParentMatches(keep, excludes)
		if err != nil {
			return nil, fmt.Errorf("invalid pattern in %s: %w", name, err)
		}

		if excluded {
			excludes = append(excludes, "!"+keep)
		}
	}

	return excludes, nil
}