package engine

import (
	"bytes"
	"errors"
	"io"
	"io/fs"
	"os"
	"path/filepath"
	"reflect"
	"sync/atomic"
)

const (
	maxSourceBytes = 8 << 20
	maxTotalBytes  = 64 << 20
)

// Source is an opaque configuration input. Construct sources with File,
// Filesystem.File, Bytes, Reader, or NoSource; the zero value is invalid.
type Source struct {
	kind      sourceKind
	path      string
	label     string
	namespace *nativeNamespace
	data      []byte
	reader    *readerSource
	createErr sourceCreationError
}

type sourceKind uint8

const (
	sourceInvalid sourceKind = iota
	sourceFile
	sourceFilesystem
	sourceBytes
	sourceReader
	sourceNone
)

type sourceCreationError uint8

const (
	creationValid sourceCreationError = iota
	creationInvalidPath
	creationInvalidLabel
	creationInvalidReader
	creationTooLarge
	creationInvalidFilesystem
	creationInvalidFilesystemLabel
	creationInvalidFilesystemPath
)

// Filesystem is a reusable opaque namespace for caller-owned fs.FS inputs.
// Copies preserve the namespace identity used for explicit contexts.
type Filesystem struct {
	namespace *nativeNamespace
	createErr sourceCreationError
}

// NewFilesystem records a borrowed filesystem without opening it. Invalid
// arguments are reported when a File source is consumed.
func NewFilesystem(label string, filesystem fs.FS) Filesystem {
	result := Filesystem{}
	if !validOverrideLabel(label) {
		result.createErr = creationInvalidFilesystemLabel
	} else if nilFilesystem(filesystem) {
		result.createErr = creationInvalidFilesystem
	} else {
		result.namespace = newNativeNamespace(label, filesystem)
	}
	return result
}

// File selects a non-root slash-relative file within this namespace.
func (f Filesystem) File(name string) Source {
	if f.createErr != creationValid {
		return Source{kind: sourceFilesystem, createErr: f.createErr}
	}
	return filesystemFile(f.namespace, name)
}

func nilFilesystem(filesystem fs.FS) bool {
	if filesystem == nil {
		return true
	}
	value := reflect.ValueOf(filesystem)
	switch value.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
		return value.IsNil()
	}
	return false
}

type readerSource struct {
	input io.Reader
	used  atomic.Bool
}

// File returns a source that reopens path for every load attempt.
func File(path string) Source {
	if path == "" {
		return Source{kind: sourceFile, createErr: creationInvalidPath}
	}
	return Source{kind: sourceFile, path: path, label: path}
}

func filesystemFile(namespace *nativeNamespace, name string) Source {
	if namespace == nil || nilFilesystem(namespace.filesystem) {
		return Source{kind: sourceFilesystem, createErr: creationInvalidFilesystem}
	}
	if name == "." || !fs.ValidPath(name) {
		return Source{kind: sourceFilesystem, createErr: creationInvalidFilesystemPath}
	}
	return Source{kind: sourceFilesystem, path: name, label: namespace.label + "/" + name, namespace: namespace}
}

// Bytes snapshots data immediately. Label must be nonempty and safe to display;
// constructor problems are reported when the source is consumed.
func Bytes(label string, data []byte) Source {
	source := Source{kind: sourceBytes, label: label}
	if label == "" {
		source.createErr = creationInvalidLabel
		return source
	}
	if len(data) > maxSourceBytes {
		source.createErr = creationTooLarge
		return source
	}
	source.data = bytes.Clone(data)
	return source
}

// Reader returns a single-use source. ConfigKit consumes input but never closes it.
// Copies of the returned Source share the same single-use claim.
func Reader(label string, input io.Reader) Source {
	source := Source{kind: sourceReader, label: label}
	if label == "" {
		source.createErr = creationInvalidLabel
		return source
	}
	if nilReader(input) {
		source.createErr = creationInvalidReader
		return source
	}
	source.reader = &readerSource{input: input}
	return source
}

// NoSource explicitly selects a defaults-only input. It creates no source record.
func NoSource() Source {
	return Source{kind: sourceNone}
}

type sourceLimits struct {
	perSource int64
	remaining int64
}

func defaultSourceLimits() sourceLimits {
	return sourceLimits{perSource: maxSourceBytes, remaining: maxTotalBytes}
}

type sourceRead struct {
	data []byte
	info *SourceInfo
}

func readSource(source Source, sourceID string, limits sourceLimits) (sourceRead, []Diagnostic) {
	if diagnostic := sourceConstructorDiagnostic(source); diagnostic != nil {
		return sourceRead{}, []Diagnostic{*diagnostic}
	}
	if source.kind == sourceNone {
		return sourceRead{}, nil
	}

	limit := min(limits.perSource, limits.remaining)
	if limit < 0 {
		limit = 0
	}
	info := &SourceInfo{ID: sourceID, Label: source.label, Layer: LayerBase, Kind: sourceSourceKind(source.kind)}

	var input io.Reader
	switch source.kind {
	case sourceFile:
		absolute, err := filepath.Abs(source.path)
		if err != nil {
			return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceRead, sourceID, "source path could not be resolved")}
		}
		info.Path = filepath.Clean(absolute)
		canonical, err := filepath.EvalSymlinks(info.Path)
		if err != nil {
			return sourceRead{info: info}, []Diagnostic{classifySourceError(err, sourceID)}
		}
		info.Path = canonical
		data, failure := readRegularFile(canonical, os.Stat, os.Open, limit)
		if failure != nil {
			return sourceRead{info: info}, []Diagnostic{sourceReadDiagnostic(failure, sourceID, CodeSourceLimitExceeded, "source exceeds the configured byte limit")}
		}
		return sourceRead{data: bytes.Clone(data), info: info}, nil
	case sourceFilesystem:
		access := newNativeAccess(source.namespace.filesystem, ".", source.namespace.token)
		info.Path = source.path
		canonical, err := access.canonical(source.path)
		if err != nil {
			return sourceRead{info: info}, []Diagnostic{classifySourceError(err, sourceID)}
		}
		info.Path = canonical
		data, failure := readRegularFile(canonical, access.stat, access.open, limit)
		if failure != nil {
			return sourceRead{info: info}, []Diagnostic{sourceReadDiagnostic(failure, sourceID, CodeSourceLimitExceeded, "source exceeds the configured byte limit")}
		}
		return sourceRead{data: bytes.Clone(data), info: info}, nil
	case sourceBytes:
		input = bytes.NewReader(source.data)
	case sourceReader:
		if !source.reader.used.CompareAndSwap(false, true) {
			return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceReaderUsed, sourceID, "reader source has already been consumed")}
		}
		input = source.reader.input
	default:
		return sourceRead{info: info}, []Diagnostic{sourceDiagnostic(CodeSourceInvalid, sourceID, "source is invalid")}
	}
	data, failure := readBoundedSource(input, limit)
	if failure != nil {
		return sourceRead{info: info}, []Diagnostic{sourceReadDiagnostic(failure, sourceID, CodeSourceLimitExceeded, "source exceeds the configured byte limit")}
	}
	return sourceRead{data: bytes.Clone(data), info: info}, nil
}

func sourceConstructorDiagnostic(source Source) *Diagnostic {
	var message string
	code := CodeSourceInvalid
	switch source.createErr {
	case creationValid:
		if source.kind != sourceInvalid {
			return nil
		}
		message = "source is invalid"
	case creationInvalidPath:
		message = "source path must not be empty"
	case creationInvalidLabel:
		message = "source label must not be empty"
	case creationInvalidReader:
		message = "source reader must not be nil"
	case creationTooLarge:
		code = CodeSourceLimitExceeded
		message = "source exceeds the configured byte limit"
	case creationInvalidFilesystem:
		message = "filesystem must not be nil"
	case creationInvalidFilesystemLabel:
		message = "filesystem label must be nonempty and safe to display"
	case creationInvalidFilesystemPath:
		message = "filesystem source path must be a non-root slash-relative valid path"
	}
	diagnostic := sourceDiagnostic(code, "", message)
	return &diagnostic
}

func sourceSourceKind(kind sourceKind) SourceKind {
	switch kind {
	case sourceFile:
		return SourceKindFile
	case sourceFilesystem:
		return SourceKindFilesystem
	case sourceBytes:
		return SourceKindBytes
	case sourceReader:
		return SourceKindReader
	default:
		return ""
	}
}

func classifySourceError(err error, sourceID string) Diagnostic {
	switch {
	case errors.Is(err, fs.ErrNotExist):
		return sourceDiagnostic(CodeSourceMissing, sourceID, "source does not exist")
	case errors.Is(err, fs.ErrPermission):
		return sourceDiagnostic(CodeSourcePermission, sourceID, "source cannot be accessed")
	default:
		return sourceDiagnostic(CodeSourceRead, sourceID, "source could not be read")
	}
}

func sourceDiagnostic(code Code, sourceID, message string) Diagnostic {
	diagnostic := Diagnostic{Code: code, Severity: SeverityError, Message: message}
	if sourceID != "" {
		diagnostic.Primary = &Location{SourceID: sourceID}
	}
	return diagnostic
}

func nilReader(input io.Reader) bool {
	if input == nil {
		return true
	}
	value := reflect.ValueOf(input)
	switch value.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
		return value.IsNil()
	default:
		return false
	}
}