package tokenize

import (
	"fmt"

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

// API holds the internal state of a tokenizer run and provides an API that
// tokenize.Handler functions can use to:
//
// • read and accept runes from the input (NextRune, Accept)
//
// • fork the API for easy lookahead support (Fork, Merge, Reset, Dispose)
//
// • flush already read input data when not needed anymore (FlushInput)
//
// • retrieve the tokenizer Result struct (Result) to read or modify the results
//
// 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 Result().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 {
	state      *apiState // shared API state data
	stackLevel int       // the stack level for this API object
}

type apiState struct {
	reader *read.Buffer
	stack  []Result // the stack, used for forking / merging the API.
}

// initialAPIstackDepth determines the initial stack depth for th API.
// This value should work in most cases. When a parser requires a higher
// stack depth, then this is no problem. The API will automatically scale
// the stack when forking beyond this default number of stack levels.
const initialAPIstackDepth = 10

// 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 {
	stack := make([]Result, 1, initialAPIstackDepth)
	stack[0] = newResult()
	state := apiState{
		reader: read.New(input),
		stack:  stack,
	}
	api := API{
		state:      &state,
		stackLevel: 0,
	}
	return api
}

// NextRune returns the rune at the current read offset.
//
// When an invalid UTF8 rune is encountered on the input, it is replaced with
// the utf.RuneError rune. It's up to the caller to handle this as an error
// when needed.
//
// After reading a rune it must be Accept()-ed to move the read cursor forward
// to the next rune. Doing so is mandatory. When doing a second call to NextRune()
// without explicitly accepting, this method will panic. You can see this as a
// built-in unit test, enforcing correct serialization of API method calls.
func (i *API) NextRune() (rune, error) {
	if i.stackLevel > len(i.state.stack)-1 {
		callerPanic("NextRune", "tokenize.API.{name}(): {name}() called at {caller} "+
			"using a non-active API fork (a parent was read or merged, causing this "+
			"fork to be invalidated)")
	}
	result := &(i.state.stack[i.stackLevel])
	if result.lastRune != nil {
		callerPanic("NextRune", "tokenize.API.{name}(): {name}() called at {caller} "+
			"without a prior call to Accept()")
	}

	readRune, err := i.state.reader.RuneAt(result.offset)
	result.lastRune = &runeInfo{r: readRune, err: err}
	return readRune, err
}

// Accept the last rune as read by NextRune() into the Result runes and move
// the cursor forward.
//
// It is not allowed to call Accept() when the previous call to NextRune()
// returned an error. Calling Accept() in such case will result in a panic.
func (i *API) Accept() {
	if i.stackLevel > len(i.state.stack)-1 {
		callerPanic("NextRune", "tokenize.API.{name}(): {name}() called at {caller} "+
			"using a non-active API fork (a parent was read or merged, causing this "+
			"fork to be invalidated)")
	}
	result := &(i.state.stack[i.stackLevel])
	if result.lastRune == nil {
		callerPanic("Accept", "tokenize.API.{name}(): {name}() called at {caller} without first calling NextRune()")
	} else if result.lastRune.err != nil {
		callerPanic("Accept", "tokenize.API.{name}(): {name}() called at {caller}, but the prior call to NextRune() failed")
	}
	result.runes = append(result.runes, result.lastRune.r)
	result.cursor.moveByRune(result.lastRune.r)
	result.offset++
	result.lastRune = nil
}

// Fork forks off a child of the API struct. It will reuse the same
// read buffer and cursor position, but for the rest this is 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 (i *API) Fork() API {
	if i.stackLevel > len(i.state.stack)-1 {
		callerPanic("NextRune", "tokenize.API.{name}(): {name}() called at {caller} "+
			"using a non-active API fork (a parent was read or merged, causing this "+
			"fork to be invalidated)")
	}
	result := &(i.state.stack[i.stackLevel])

	// Grow the stack storage when needed.
	newStackSize := i.stackLevel + 2
	if cap(i.state.stack) < newStackSize {
		newStack := make([]Result, newStackSize, 2*newStackSize)
		copy(newStack, i.state.stack)
		i.state.stack = newStack

	}

	// Create the new fork.
	child := API{
		state:      i.state,
		stackLevel: i.stackLevel + 1,
	}
	childResult := newResult()
	childResult.cursor = result.cursor
	childResult.offset = result.offset
	i.state.stack = i.state.stack[:newStackSize] // todo use append() directly?
	i.state.stack[child.stackLevel] = childResult

	// Update the parent.
	result.lastRune = nil

	return child
}

// stackDump provides a dump of the currently active stack levels in the API.
// This is used for debugging purposes and is normally not part of the standard
// code flow.
func (i *API) stackDump() {
	for i, r := range i.state.stack {
		fmt.Printf("[%d] %s: %q\n", i, r.cursor, r.String())
	}
}

// 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.
func (i *API) Merge() {
	if i.stackLevel == 0 {
		callerPanic("Merge", "tokenize.API.{name}(): {name}() called at {caller} on a non-forked API")
	}
	if i.stackLevel > len(i.state.stack)-1 {
		callerPanic("NextRune", "tokenize.API.{name}(): {name}() called at {caller} "+
			"using a non-active API fork (a parent was read or merged, causing this "+
			"fork to be invalidated)")
	}
	result := &(i.state.stack[i.stackLevel])
	parentResult := &(i.state.stack[i.stackLevel-1])
	parentResult.runes = append(parentResult.runes, result.runes...)
	parentResult.tokens = append(parentResult.tokens, result.tokens...)
	parentResult.offset = result.offset
	parentResult.cursor = result.cursor
	i.Reset()
	i.DisposeChilds()
}

func (i *API) Dispose() {
	i.state.stack = i.state.stack[:i.stackLevel]
}

func (i *API) DisposeChilds() {
	i.state.stack = i.state.stack[:i.stackLevel+1]
}

func (i *API) Reset() {
	result := &(i.state.stack[i.stackLevel])
	result.lastRune = nil
	result.runes = result.runes[:0]
	result.tokens = result.tokens[:0]
	result.err = nil
}

// FlushInput flushes processed input data from the read.Buffer.
// In this context 'processed' means all runes that were read using NextRune()
// and that were added to the results using Accept().
//
// Note:
// When writing your own TokenHandler, you normally won't have to call this
// method yourself. It is automatically called by parsekit when needed.
func (i API) FlushInput() bool {
	result := &(i.state.stack[i.stackLevel])
	if result.offset > 0 {
		i.state.reader.Flush(result.offset)
		result.offset = 0
		return true
	}
	return false
}

// Result returns the Result struct from the API. The returned struct
// can be used to retrieve and to modify result data.
func (i API) Result() *Result {
	return &(i.state.stack[i.stackLevel])
}