package engine

import (
	"errors"
	"io"
	"io/fs"
	"os"
)

type sourceReadFailureKind uint8

const (
	sourceReadIO sourceReadFailureKind = iota
	sourceReadNotRegular
	sourceReadLimit
)

// sourceReadFailure never escapes the private source boundary. missingTarget is
// set only for preflight/open failures; a later missing-file error is not optional.
type sourceReadFailure struct {
	kind          sourceReadFailureKind
	cause         error
	missingTarget bool
}

// readRegularFile borrows stat/open operations so root-bound callers retain their
// containment policy. Preflight rejects FIFOs before open can block. The opened
// handle is checked again and always closed; read-only close errors are ignored.
func readRegularFile(name string, stat func(string) (fs.FileInfo, error), open func(string) (*os.File, error), limit int64) ([]byte, *sourceReadFailure) {
	info, err := stat(name)
	if err != nil {
		return nil, &sourceReadFailure{cause: err, missingTarget: errors.Is(err, fs.ErrNotExist)}
	}
	if !info.Mode().IsRegular() {
		return nil, &sourceReadFailure{kind: sourceReadNotRegular}
	}
	file, err := open(name)
	if err != nil {
		return nil, &sourceReadFailure{cause: err, missingTarget: errors.Is(err, fs.ErrNotExist)}
	}
	defer func() { _ = file.Close() }()
	info, err = file.Stat()
	if err != nil {
		return nil, &sourceReadFailure{cause: err}
	}
	if !info.Mode().IsRegular() {
		return nil, &sourceReadFailure{kind: sourceReadNotRegular}
	}
	if info.Size() > limit {
		return nil, &sourceReadFailure{kind: sourceReadLimit}
	}
	return readBoundedSource(file, limit)
}

func readBoundedSource(input io.Reader, limit int64) ([]byte, *sourceReadFailure) {
	data, err := io.ReadAll(io.LimitReader(input, limit+1))
	if err != nil {
		return nil, &sourceReadFailure{cause: err}
	}
	if int64(len(data)) > limit {
		return nil, &sourceReadFailure{kind: sourceReadLimit}
	}
	return data, nil
}

func sourceReadDiagnostic(failure *sourceReadFailure, sourceID string, limitCode Code, limitMessage string) Diagnostic {
	switch failure.kind {
	case sourceReadNotRegular:
		return sourceDiagnostic(CodeSourceNotRegular, sourceID, "source is not a regular file")
	case sourceReadLimit:
		return sourceDiagnostic(limitCode, sourceID, limitMessage)
	default:
		return classifySourceError(failure.cause, sourceID)
	}
}