package tokenize

import (
	"fmt"

	"git.makaay.nl/mauricem/go-parsekit/read"
)

// API holds the internal state of a tokenizer run. A tokenizer run uses'
// tokenize.Handler functions to move the tokenizer forward through the
// input and to provide tokenizer output.
//
// The methods as provided by the API are used by tokenize.Handler functions to:
//
// • access and process runes / bytes from the input data
//
// • flush processed input data that are not required anymore (FlushInput)
//
// • fork the API for easy lookahead support (Fork, Merge, Reset, Dispose)
//
// • emit tokens and/or bytes to be used by a parser
//
// BASIC OPERATION:
//
// To retrieve the next rune from the API, call the NextRune() method.
//
// When the rune is to be accepted as input, call the method Accept(). The rune
// is then added to the result runes of the API and the read cursor is moved
// forward.
//
// By invoking NextRune() + Accept() multiple times, the result can be extended
// with as many runes as needed. Runes collected this way can later on be
// retrieved using the method Runes().
//
// It is mandatory to call Accept() after retrieving a rune, before calling
// NextRune() again. Failing to do so will result in a panic.
//
// Next to adding runes to the result, it is also possible to modify the
// stored runes or to add lexical Tokens to the result. For all things
// concerning results, take a look at the Result struct, which
// can be accessed though the method Result().
//
// FORKING OPERATION FOR EASY LOOKEAHEAD SUPPORT:
//
// Sometimes, we must be able to perform a lookahead, which might either
// succeed or fail. In case of a failing lookahead, the state of the
// API must be brought back to the original state, so we can try
// a different route.
//
// The way in which this is supported, is by forking an API struct by
// calling method Fork(). This will return a forked child API, with
// empty result data, but using the same read cursor position as the
// forked parent.
//
// After forking, the same interface as described for BASIC OPERATION can be
// used to fill the results. When the lookahead was successful, then
// Merge() can be called on the forked child to append the child's results
// to the parent's results, and to move the read cursor position to that
// of the child.
//
// When the lookahead was unsuccessful, then the forked child API can
// disposed by calling Dispose() on the forked child. This is not mandatory.
// Garbage collection will take care of this automatically.
// The parent API was never modified, so it can safely be used after disposal
// as if the lookahead never happened.
//
// Opinionized note:
// Many tokenizers/parsers take a different approach on lookaheads by using
// peeks and by moving the read cursor position back and forth, or by putting
// read input back on the input stream. That often leads to code that is
// efficient, however, in my opinion, not very intuitive to read. It can also
// be tedious to get the cursor position back at the correct position, which
// can lead to hard to track bugs. I much prefer this forking method, since
// no bookkeeping has to be implemented when implementing a parser.
type API struct {
	stackFrames  []stackFrame // the stack frames, containing stack level-specific data
	stackLevel   int          // the current stack level
	stackFrame   *stackFrame  // the current stack frame
	reader       *read.Buffer // the buffered input reader
	Byte         ByteMode     // access to a set of byte-based input methods
	Rune         RuneMode     // access to a set of rune-based input methods
	Output       Output       // provides output-related functionality
	outputTokens []Token      // accepted tokens
	outputData   []byte       // accepted data
}

type stackFrame struct {
	offset     int // the read offset (relative to the start of the reader buffer) for this stack frame
	column     int // the column at which the cursor is (0-indexed)
	line       int // the line at which the cursor is (0-indexed)
	bytesStart int // the starting point in the API.bytes slice for runes produced by this stack level
	bytesEnd   int // the end point in the API.bytes slice for runes produced by this stack level
	tokenStart int // the starting point in the API.tokens slice for tokens produced by this stack level
	tokenEnd   int // the end point in the API.tokens slice for tokens produced by this stack level

	// TODO
	err error // can be used by a Handler to report a specific issue with the input
}

const initialStackDepth = 64
const initialTokenStoreLength = 64
const initialByteStoreLength = 1024

// NewAPI initializes a new API struct, wrapped around the provided input.
// For an overview of allowed inputs, take a look at the documentation
// for parsekit.read.New().
func NewAPI(input interface{}) *API {
	api := &API{
		reader:      read.New(input),
		stackFrames: make([]stackFrame, initialStackDepth),
	}
	api.Byte = ByteMode{api: api}
	api.Rune = RuneMode{api: api}
	api.Output = Output{api: api}
	api.stackFrame = &api.stackFrames[0]

	return api
}

// Fork forks off a child of the API struct. It will reuse the same
// read buffer and cursor position, but for the rest this can be considered
// a fresh API.
//
// By forking an API, you can freely work with the forked child, without
// affecting the parent API. This is for example useful when you must perform
// some form of lookahead.
//
// When processing of the Handler was successful and you want to add the results
// to the parent API, you can call Merge() on the forked child.
// This will add the results to the results of the parent (runes, tokens).
// It also updates the read cursor position of the parent to that of the child.
//
// When the lookahead was unsuccessful, then the forked child API can
// disposed by calling Dispose() on the forked child. This is not mandatory.
// Garbage collection will take care of this automatically.
// The parent API was never modified, so it can safely be used after disposal
// as if the lookahead never happened.
func (tokenAPI *API) Fork() int {
	tokenAPI.stackLevel++
	newStackLevel := tokenAPI.stackLevel

	// Grow the stack frames capacity when needed.
	frames := tokenAPI.stackFrames
	if cap(frames) < (newStackLevel + 1) {
		newFrames := make([]stackFrame, (newStackLevel+1)*2)
		copy(newFrames, frames)
		frames = newFrames
	}

	// This can be written in a shorter way, but this turned out to
	// be the best way performance-wise.
	parent := tokenAPI.stackFrame
	child := &frames[newStackLevel]
	child.offset = parent.offset
	child.column = parent.column
	child.line = parent.line
	child.bytesStart = parent.bytesEnd
	child.bytesEnd = parent.bytesEnd
	child.tokenStart = parent.tokenEnd
	child.tokenEnd = parent.tokenEnd
	tokenAPI.stackFrame = child

	return newStackLevel
}

// Merge appends the results of a forked child API (runes, tokens) to the
// results of its parent. The read cursor of the parent is also updated
// to that of the forked child.
//
// After the merge operation, the child results are reset so it can immediately
// be reused for performing another match. This means that all Result data are
// cleared, but the read cursor position is kept at its current position.
// This allows a child to feed results in chunks to its parent.
//
// Once the child is no longer needed, it can be disposed of by using the
// method Dispose(), which will return the tokenizer to the parent.
func (tokenAPI *API) Merge(stackLevel int) {
	if stackLevel == 0 {
		callerPanic("Merge", "tokenize.API.{name}(): {name}() called at {caller} "+
			"on the top-level API stack level 0")
	}
	if stackLevel != tokenAPI.stackLevel {
		callerPanic("Merge", "tokenize.API.{name}(): {name}() called at {caller} "+
			"on API stack level %d, but the current stack level is %d "+
			"(forgot to Dispose() a forked child?)", stackLevel, tokenAPI.stackLevel)
	}

	parent := &tokenAPI.stackFrames[stackLevel-1]
	f := tokenAPI.stackFrame

	// The end of the parent slice aligns with the start of the child slice.
	// Because of this, to merge the parent slice can simply be expanded
	// to include the child slice.
	// parent :  |----------|
	// child:               |------|
	// After merge operation:
	// parent:   |-----------------|
	// child:                      |---> continue reading from here
	parent.bytesEnd = f.bytesEnd
	f.bytesStart = f.bytesEnd

	// The same logic applies to tokens.
	parent.tokenEnd = f.tokenEnd
	f.tokenStart = f.tokenEnd

	parent.offset = f.offset
	parent.line = f.line
	parent.column = f.column

	f.err = nil
}

// Reset moves the input cursor back to the beginning for the currently active API child.
// Aditionally, any output (bytes and tokens) that was emitted from the API child are
// cleared as well.
func (api *API) Reset() {
	f := api.stackFrame
	if api.stackLevel == 0 {
		f.column = 0
		f.line = 0
		f.offset = 0
	} else {
		parent := api.stackFrames[api.stackLevel-1]
		f.column = parent.column
		f.line = parent.line
		f.offset = parent.offset
	}
	f.bytesEnd = f.bytesStart
	f.tokenEnd = f.tokenStart
	f.err = nil
}

func (tokenAPI *API) Dispose(stackLevel int) {
	if stackLevel == 0 {
		callerPanic("Dispose", "tokenize.API.{name}(): {name}() called at {caller} "+
			"on the top-level API stack level 0")
	}
	if stackLevel != tokenAPI.stackLevel {
		callerPanic("Dispose", "tokenize.API.{name}(): {name}() called at {caller} "+
			"on API stack level %d, but the current stack level is %d "+
			"(forgot to Dispose() a forked child?)", stackLevel, tokenAPI.stackLevel)
	}

	tokenAPI.stackLevel = stackLevel - 1
	tokenAPI.stackFrame = &tokenAPI.stackFrames[stackLevel-1]
}

// FlushInput flushes input data from the read.Buffer up to the current
// read offset of the parser.
//
// Note:
// When writing your own TokenHandler, you normally won't have to call this
// method yourself. It is automatically called by parsekit when possible.
func (api *API) FlushInput() bool {
	if api.stackFrame.offset > 0 {
		api.reader.Flush(api.stackFrame.offset)
		api.stackFrame.offset = 0
		return true
	}
	return false
}

func (api *API) Cursor() string {
	if api.stackFrame.line == 0 && api.stackFrame.column == 0 {
		return fmt.Sprintf("start of file")
	}
	return fmt.Sprintf("line %d, column %d", api.stackFrame.line+1, api.stackFrame.column+1)
}