#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <fcntl.h>
#include <errno.h>
#include <wchar.h>
#include <stdint.h>
#include <libgen.h>
#include <limits.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>

#include "text.h"
#include "text-util.h"
#include "text-motions.h"
#include "util.h"
#include "array.h"
#include "text-internal.h"

/* A piece holds a reference (but doesn't itself store) a certain amount of data.
 * All active pieces chained together form the whole content of the document.
 * At the beginning there exists only one piece, spanning the whole document.
 * Upon insertion/deletion new pieces will be created to represent the changes.
 * Generally pieces are never destroyed, but kept around to perform undo/redo
 * operations.
 */
struct Piece {
	Text *text;             /* text to which this piece belongs */
	Piece *prev, *next;     /* pointers to the logical predecessor/successor */
	Piece *global_prev;     /* double linked list in order of allocation, */
	Piece *global_next;     /* used to free individual pieces */
	const char *data;       /* pointer into a Block holding the data */
	size_t len;             /* the length in number of bytes of the data */
};

/* used to transform a global position (byte offset starting from the beginning
 * of the text) into an offset relative to a piece.
 */
typedef struct {
	Piece *piece;           /* piece holding the location */
	size_t off;             /* offset into the piece in bytes */
} Location;

/* A Span holds a certain range of pieces. Changes to the document are always
 * performed by swapping out an existing span with a new one.
 */
typedef struct {
	Piece *start, *end;     /* start/end of the span */
	size_t len;             /* the sum of the lengths of the pieces which form this span */
} Span;

/* A Change keeps all needed information to redo/undo an insertion/deletion. */
typedef struct Change Change;
struct Change {
	Span old;               /* all pieces which are being modified/swapped out by the change */
	Span new;               /* all pieces which are introduced/swapped in by the change */
	size_t pos;             /* absolute position at which the change occured */
	Change *next;           /* next change which is part of the same revision */
	Change *prev;           /* previous change which is part of the same revision */
};

/* A Revision is a list of Changes which are used to undo/redo all modifications
 * since the last snapshot operation. Revisions are stored in a directed graph structure.
 */
typedef struct Revision Revision;
struct Revision {
	Change *change;         /* the most recent change */
	Revision *next;         /* the next (child) revision in the undo tree */
	Revision *prev;         /* the previous (parent) revision in the undo tree */
	Revision *earlier;      /* the previous Revision, chronologically */
	Revision *later;        /* the next Revision, chronologically */
	time_t time;            /* when the first change of this revision was performed */
	size_t seq;             /* a unique, strictly increasing identifier */
};

typedef struct {
	size_t pos;             /* position in bytes from start of file */
	size_t lineno;          /* line number in file i.e. number of '\n' in [0, pos) */
} LineCache;

/* The main struct holding all information of a given file */
struct Text {
	Array blocks;           /* blocks which hold text content */
	Piece *pieces;          /* all pieces which have been allocated, used to free them */
	Piece *cache;           /* most recently modified piece */
	Piece begin, end;       /* sentinel nodes which always exists but don't hold any data */
	Revision *history;        /* undo tree */
	Revision *current_revision; /* revision holding all file changes until a snapshot is performed */
	Revision *last_revision;    /* the last revision added to the tree, chronologically */
	Revision *saved_revision;   /* the last revision at the time of the save operation */
	size_t size;            /* current file content size in bytes */
	struct stat info;       /* stat as probed at load time */
	LineCache lines;        /* mapping between absolute pos in bytes and logical line breaks */
};

/* block management */
static const char *block_store(Text*, const char *data, size_t len);
/* cache layer */
static void cache_piece(Text *txt, Piece *p);
static bool cache_contains(Text *txt, Piece *p);
static bool cache_insert(Text *txt, Piece *p, size_t off, const char *data, size_t len);
static bool cache_delete(Text *txt, Piece *p, size_t off, size_t len);
/* piece management */
static Piece *piece_alloc(Text *txt);
static void piece_free(Piece *p);
static void piece_init(Piece *p, Piece *prev, Piece *next, const char *data, size_t len);
static Location piece_get_intern(Text *txt, size_t pos);
static Location piece_get_extern(const Text *txt, size_t pos);
/* span management */
static void span_init(Span *span, Piece *start, Piece *end);
static void span_swap(Text *txt, Span *old, Span *new);
/* change management */
static Change *change_alloc(Text *txt, size_t pos);
static void change_free(Change *c);
/* revision management */
static Revision *revision_alloc(Text *txt);
static void revision_free(Revision *rev);
/* logical line counting cache */
static void lineno_cache_invalidate(LineCache *cache);
static size_t lines_skip_forward(Text *txt, size_t pos, size_t lines, size_t *lines_skiped);
static size_t lines_count(Text *txt, size_t pos, size_t len);

/* stores the given data in a block, allocates a new one if necessary. Returns
 * a pointer to the storage location or NULL if allocation failed. */
static const char *block_store(Text *txt, const char *data, size_t len) {
	Block *blk = array_get_ptr(&txt->blocks, array_length(&txt->blocks)-1);
	if (!blk || !block_capacity(blk, len)) {
		blk = block_alloc(len);
		if (!blk)
			return NULL;
		if (!array_add_ptr(&txt->blocks, blk)) {
			block_free(blk);
			return NULL;
		}
	}
	return block_append(blk, data, len);
}

/* cache the given piece if it is the most recently changed one */
static void cache_piece(Text *txt, Piece *p) {
	Block *blk = array_get_ptr(&txt->blocks, array_length(&txt->blocks)-1);
	if (!blk || p->data < blk->data || p->data + p->len != blk->data + blk->len)
		return;
	txt->cache = p;
}

/* check whether the given piece was the most recently modified one */
static bool cache_contains(Text *txt, Piece *p) {
	Block *blk = array_get_ptr(&txt->blocks, array_length(&txt->blocks)-1);
	Revision *rev = txt->current_revision;
	if (!blk || !txt->cache || txt->cache != p || !rev || !rev->change)
		return false;

	Piece *start = rev->change->new.start;
	Piece *end = rev->change->new.end;
	bool found = false;
	for (Piece *cur = start; !found; cur = cur->next) {
		if (cur == p)
			found = true;
		if (cur == end)
			break;
	}

	return found && p->data + p->len == blk->data + blk->len;
}

/* try to insert a chunk of data at a given piece offset. The insertion is only
 * performed if the piece is the most recently changed one. The length of the
 * piece, the span containing it and the whole text is adjusted accordingly */
static bool cache_insert(Text *txt, Piece *p, size_t off, const char *data, size_t len) {
	if (!cache_contains(txt, p))
		return false;
	Block *blk = array_get_ptr(&txt->blocks, array_length(&txt->blocks)-1);
	size_t bufpos = p->data + off - blk->data;
	if (!block_insert(blk, bufpos, data, len))
		return false;
	p->len += len;
	txt->current_revision->change->new.len += len;
	txt->size += len;
	return true;
}

/* try to delete a chunk of data at a given piece offset. The deletion is only
 * performed if the piece is the most recently changed one and the whole
 * affected range lies within it. The length of the piece, the span containing it
 * and the whole text is adjusted accordingly */
static bool cache_delete(Text *txt, Piece *p, size_t off, size_t len) {
	if (!cache_contains(txt, p))
		return false;
	Block *blk = array_get_ptr(&txt->blocks, array_length(&txt->blocks)-1);
	size_t end;
	size_t bufpos = p->data + off - blk->data;
	if (!addu(off, len, &end) || end > p->len || !block_delete(blk, bufpos, len))
		return false;
	p->len -= len;
	txt->current_revision->change->new.len -= len;
	txt->size -= len;
	return true;
}

/* initialize a span and calculate its length */
static void span_init(Span *span, Piece *start, Piece *end) {
	size_t len = 0;
	span->start = start;
	span->end = end;
	for (Piece *p = start; p; p = p->next) {
		len += p->len;
		if (p == end)
			break;
	}
	span->len = len;
}

/* swap out an old span and replace it with a new one.
 *
 *  - if old is an empty span do not remove anything, just insert the new one
 *  - if new is an empty span do not insert anything, just remove the old one
 *
 * adjusts the document size accordingly.
 */
static void span_swap(Text *txt, Span *old, Span *new) {
	if (old->len == 0 && new->len == 0) {
		return;
	} else if (old->len == 0) {
		/* insert new span */
		new->start->prev->next = new->start;
		new->end->next->prev = new->end;
	} else if (new->len == 0) {
		/* delete old span */
		old->start->prev->next = old->end->next;
		old->end->next->prev = old->start->prev;
	} else {
		/* replace old with new */
		old->start->prev->next = new->start;
		old->end->next->prev = new->end;
	}
	txt->size -= old->len;
	txt->size += new->len;
}

/* Allocate a new revision and place it in the revision graph.
 * All further changes will be associated with this revision. */
static Revision *revision_alloc(Text *txt) {
	Revision *rev = calloc(1, sizeof *rev);
	if (!rev)
		return NULL;
	rev->time = time(NULL);
	txt->current_revision = rev;

	/* set sequence number */
	if (!txt->last_revision)
		rev->seq = 0;
	else
		rev->seq = txt->last_revision->seq + 1;

	/* set earlier, later pointers */
	if (txt->last_revision)
		txt->last_revision->later = rev;
	rev->earlier = txt->last_revision;

	if (!txt->history) {
		txt->history = rev;
		return rev;
	}

	/* set prev, next pointers */
	rev->prev = txt->history;
	txt->history->next = rev;
	txt->history = rev;
	return rev;
}

static void revision_free(Revision *rev) {
	if (!rev)
		return;
	for (Change *next, *c = rev->change; c; c = next) {
		next = c->next;
		change_free(c);
	}
	free(rev);
}

static Piece *piece_alloc(Text *txt) {
	Piece *p = calloc(1, sizeof *p);
	if (!p)
		return NULL;
	p->text = txt;
	p->global_next = txt->pieces;
	if (txt->pieces)
		txt->pieces->global_prev = p;
	txt->pieces = p;
	return p;
}

static void piece_free(Piece *p) {
	if (!p)
		return;
	if (p->global_prev)
		p->global_prev->global_next = p->global_next;
	if (p->global_next)
		p->global_next->global_prev = p->global_prev;
	if (p->text->pieces == p)
		p->text->pieces = p->global_next;
	if (p->text->cache == p)
		p->text->cache = NULL;
	free(p);
}

static void piece_init(Piece *p, Piece *prev, Piece *next, const char *data, size_t len) {
	p->prev = prev;
	p->next = next;
	p->data = data;
	p->len = len;
}

/* returns the piece holding the text at byte offset pos. If pos happens to
 * be at a piece boundary i.e. the first byte of a piece then the previous piece
 * to the left is returned with an offset of piece->len. This is convenient for
 * modifications to the piece chain where both pieces (the returned one and the
 * one following it) are needed, but unsuitable as a public interface.
 *
 * in particular if pos is zero, the begin sentinel piece is returned.
 */
static Location piece_get_intern(Text *txt, size_t pos) {
	size_t cur = 0;
	for (Piece *p = &txt->begin; p->next; p = p->next) {
		if (cur <= pos && pos <= cur + p->len)
			return (Location){ .piece = p, .off = pos - cur };
		cur += p->len;
	}

	return (Location){ 0 };
}

/* similiar to piece_get_intern but usable as a public API. Returns the piece
 * holding the text at byte offset pos. Never returns a sentinel piece.
 * it pos is the end of file (== text_size()) and the file is not empty then
 * the last piece holding data is returned.
 */
static Location piece_get_extern(const Text *txt, size_t pos) {
	size_t cur = 0;
	Piece *p;

	for (p = txt->begin.next; p->next; p = p->next) {
		if (cur <= pos && pos < cur + p->len)
			return (Location){ .piece = p, .off = pos - cur };
		cur += p->len;
	}

	if (cur == pos)
		return (Location){ .piece = p->prev, .off = p->prev->len };

	return (Location){ 0 };
}

/* allocate a new change, associate it with current revision or a newly
 * allocated one if none exists. */
static Change *change_alloc(Text *txt, size_t pos) {
	Revision *rev = txt->current_revision;
	if (!rev) {
		rev = revision_alloc(txt);
		if (!rev)
			return NULL;
	}
	Change *c = calloc(1, sizeof *c);
	if (!c)
		return NULL;
	c->pos = pos;
	c->next = rev->change;
	if (rev->change)
		rev->change->prev = c;
	rev->change = c;
	return c;
}

static void change_free(Change *c) {
	if (!c)
		return;
	/* only free the new part of the span, the old one is still in use */
	if (c->new.start != c->new.end)
		piece_free(c->new.end);
	piece_free(c->new.start);
	free(c);
}

/* When inserting new data there are 2 cases to consider.
 *
 *  - in the first the insertion point falls into the middle of an exisiting
 *    piece which is replaced by three new pieces:
 *
 *      /-+ --> +---------------+ --> +-\
 *      | |     | existing text |     | |
 *      \-+ <-- +---------------+ <-- +-/
 *                         ^
 *                         Insertion point for "demo "
 *
 *      /-+ --> +---------+ --> +-----+ --> +-----+ --> +-\
 *      | |     | existing|     |demo |     |text |     | |
 *      \-+ <-- +---------+ <-- +-----+ <-- +-----+ <-- +-/
 *
 *  - the second case deals with an insertion point at a piece boundry:
 *
 *      /-+ --> +---------------+ --> +-\
 *      | |     | existing text |     | |
 *      \-+ <-- +---------------+ <-- +-/
 *            ^
 *            Insertion point for "short"
 *
 *      /-+ --> +-----+ --> +---------------+ --> +-\
 *      | |     |short|     | existing text |     | |
 *      \-+ <-- +-----+ <-- +---------------+ <-- +-/
 */
bool text_insert(Text *txt, size_t pos, const char *data, size_t len) {
	if (len == 0)
		return true;
	if (pos > txt->size)
		return false;
	if (pos < txt->lines.pos)
		lineno_cache_invalidate(&txt->lines);

	Location loc = piece_get_intern(txt, pos);
	Piece *p = loc.piece;
	if (!p)
		return false;
	size_t off = loc.off;
	if (cache_insert(txt, p, off, data, len))
		return true;

	Change *c = change_alloc(txt, pos);
	if (!c)
		return false;

	if (!(data = block_store(txt, data, len)))
		return false;

	Piece *new = NULL;

	if (off == p->len) {
		/* insert between two existing pieces, hence there is nothing to
		 * remove, just add a new piece holding the extra text */
		if (!(new = piece_alloc(txt)))
			return false;
		piece_init(new, p, p->next, data, len);
		span_init(&c->new, new, new);
		span_init(&c->old, NULL, NULL);
	} else {
		/* insert into middle of an existing piece, therefore split the old
		 * piece. That is we have 3 new pieces one containing the content
		 * before the insertion point then one holding the newly inserted
		 * text and one holding the content after the insertion point.
		 */
		Piece *before = piece_alloc(txt);
		new = piece_alloc(txt);
		Piece *after = piece_alloc(txt);
		if (!before || !new || !after)
			return false;
		piece_init(before, p->prev, new, p->data, off);
		piece_init(new, before, after, data, len);
		piece_init(after, new, p->next, p->data + off, p->len - off);

		span_init(&c->new, before, after);
		span_init(&c->old, p, p);
	}

	cache_piece(txt, new);
	span_swap(txt, &c->old, &c->new);
	return true;
}

static size_t revision_undo(Text *txt, Revision *rev) {
	size_t pos = EPOS;
	for (Change *c = rev->change; c; c = c->next) {
		span_swap(txt, &c->new, &c->old);
		pos = c->pos;
	}
	return pos;
}

static size_t revision_redo(Text *txt, Revision *rev) {
	size_t pos = EPOS;
	Change *c = rev->change;
	while (c->next)
		c = c->next;
	for ( ; c; c = c->prev) {
		span_swap(txt, &c->old, &c->new);
		pos = c->pos;
		if (c->new.len > c->old.len)
			pos += c->new.len - c->old.len;
	}
	return pos;
}

size_t text_undo(Text *txt) {
	size_t pos = EPOS;
	/* taking rev snapshot makes sure that txt->current_revision is reset */
	text_snapshot(txt);
	Revision *rev = txt->history->prev;
	if (!rev)
		return pos;
	pos = revision_undo(txt, txt->history);
	txt->history = rev;
	lineno_cache_invalidate(&txt->lines);
	return pos;
}

size_t text_redo(Text *txt) {
	size_t pos = EPOS;
	/* taking a snapshot makes sure that txt->current_revision is reset */
	text_snapshot(txt);
	Revision *rev = txt->history->next;
	if (!rev)
		return pos;
	pos = revision_redo(txt, rev);
	txt->history = rev;
	lineno_cache_invalidate(&txt->lines);
	return pos;
}

static bool history_change_branch(Revision *rev) {
	bool changed = false;
	while (rev->prev) {
		if (rev->prev->next != rev) {
			rev->prev->next = rev;
			changed = true;
		}
		rev = rev->prev;
	}
	return changed;
}

static size_t history_traverse_to(Text *txt, Revision *rev) {
	size_t pos = EPOS;
	if (!rev)
		return pos;
	bool changed = history_change_branch(rev);
	if (!changed) {
		if (rev->seq == txt->history->seq) {
			return txt->lines.pos;
		} else if (rev->seq > txt->history->seq) {
			while (txt->history != rev)
				pos = text_redo(txt);
			return pos;
		} else if (rev->seq < txt->history->seq) {
			while (txt->history != rev)
				pos = text_undo(txt);
			return pos;
		}
	} else {
		while (txt->history->prev && txt->history->prev->next == txt->history)
			text_undo(txt);
		pos = text_undo(txt);
		while (txt->history != rev)
			pos = text_redo(txt);
		return pos;
	}
	return pos;
}

size_t text_earlier(Text *txt) {
	return history_traverse_to(txt, txt->history->earlier);
}

size_t text_later(Text *txt) {
	return history_traverse_to(txt, txt->history->later);
}

size_t text_restore(Text *txt, time_t time) {
	Revision *rev = txt->history;
	while (time < rev->time && rev->earlier)
		rev = rev->earlier;
	while (time > rev->time && rev->later)
		rev = rev->later;
	time_t diff = labs(rev->time - time);
	if (rev->earlier && rev->earlier != txt->history && labs(rev->earlier->time - time) < diff)
		rev = rev->earlier;
	if (rev->later && rev->later != txt->history && labs(rev->later->time - time) < diff)
		rev = rev->later;
	return history_traverse_to(txt, rev);
}

time_t text_state(const Text *txt) {
	return txt->history->time;
}

Text *text_loadat_method(int dirfd, const char *filename, enum TextLoadMethod method) {
	Text *txt = calloc(1, sizeof *txt);
	if (!txt)
		return NULL;
	Piece *p = piece_alloc(txt);
	if (!p)
		goto out;
	Block *block = NULL;
	array_init(&txt->blocks);
	lineno_cache_invalidate(&txt->lines);
	if (filename) {
		errno = 0;
		block = block_load(dirfd, filename, method, &txt->info);
		if (!block && errno)
			goto out;
		if (block && !array_add_ptr(&txt->blocks, block)) {
			block_free(block);
			goto out;
		}
	}

	if (!block)
		piece_init(p, &txt->begin, &txt->end, "\0", 0);
	else
		piece_init(p, &txt->begin, &txt->end, block->data, block->len);

	piece_init(&txt->begin, NULL, p, NULL, 0);
	piece_init(&txt->end, p, NULL, NULL, 0);
	txt->size = p->len;
	/* write an empty revision */
	change_alloc(txt, EPOS);
	text_snapshot(txt);
	txt->saved_revision = txt->history;

	return txt;
out:
	text_free(txt);
	return NULL;
}

struct stat text_stat(const Text *txt) {
	return txt->info;
}

void text_saved(Text *txt, struct stat *meta) {
	if (meta)
		txt->info = *meta;
	txt->saved_revision = txt->history;
	text_snapshot(txt);
}

Block *text_block_mmaped(Text *txt) {
	Block *block = array_get_ptr(&txt->blocks, 0);
	if (block && block->type == BLOCK_TYPE_MMAP_ORIG && block->size)
		return block;
	return NULL;
}

/* A delete operation can either start/stop midway through a piece or at
 * a boundary. In the former case a new piece is created to represent the
 * remaining text before/after the modification point.
 *
 *      /-+ --> +---------+ --> +-----+ --> +-----+ --> +-\
 *      | |     | existing|     |demo |     |text |     | |
 *      \-+ <-- +---------+ <-- +-----+ <-- +-----+ <-- +-/
 *                   ^                         ^
 *                   |------ delete range -----|
 *
 *      /-+ --> +----+ --> +--+ --> +-\
 *      | |     | exi|     |t |     | |
 *      \-+ <-- +----+ <-- +--+ <-- +-/
 */
bool text_delete(Text *txt, size_t pos, size_t len) {
	if (len == 0)
		return true;
	size_t pos_end;
	if (!addu(pos, len, &pos_end) || pos_end > txt->size)
		return false;
	if (pos < txt->lines.pos)
		lineno_cache_invalidate(&txt->lines);

	Location loc = piece_get_intern(txt, pos);
	Piece *p = loc.piece;
	if (!p)
		return false;
	size_t off = loc.off;
	if (cache_delete(txt, p, off, len))
		return true;
	Change *c = change_alloc(txt, pos);
	if (!c)
		return false;

	bool midway_start = false, midway_end = false; /* split pieces? */
	Piece *before, *after; /* unmodified pieces before/after deletion point */
	Piece *start, *end;    /* span which is removed */
	size_t cur;            /* how much has already been deleted */

	if (off == p->len) {
		/* deletion starts at a piece boundary */
		cur = 0;
		before = p;
		start = p->next;
	} else {
		/* deletion starts midway through a piece */
		midway_start = true;
		cur = p->len - off;
		start = p;
		before = piece_alloc(txt);
		if (!before)
			return false;
	}

	/* skip all pieces which fall into deletion range */
	while (cur < len) {
		p = p->next;
		cur += p->len;
	}

	if (cur == len) {
		/* deletion stops at a piece boundary */
		end = p;
		after = p->next;
	} else {
		/* cur > len: deletion stops midway through a piece */
		midway_end = true;
		end = p;
		after = piece_alloc(txt);
		if (!after)
			return false;
		piece_init(after, before, p->next, p->data + p->len - (cur - len), cur - len);
	}

	if (midway_start) {
		/* we finally know which piece follows our newly allocated before piece */
		piece_init(before, start->prev, after, start->data, off);
	}

	Piece *new_start = NULL, *new_end = NULL;
	if (midway_start) {
		new_start = before;
		if (!midway_end)
			new_end = before;
	}
	if (midway_end) {
		if (!midway_start)
			new_start = after;
		new_end = after;
	}

	span_init(&c->new, new_start, new_end);
	span_init(&c->old, start, end);
	span_swap(txt, &c->old, &c->new);
	return true;
}

bool text_delete_range(Text *txt, const Filerange *r) {
	if (!text_range_valid(r))
		return false;
	return text_delete(txt, r->start, text_range_size(r));
}

/* preserve the current text content such that it can be restored by
 * means of undo/redo operations */
bool text_snapshot(Text *txt) {
	if (txt->current_revision)
		txt->last_revision = txt->current_revision;
	txt->current_revision = NULL;
	txt->cache = NULL;
	return true;
}


void text_free(Text *txt) {
	if (!txt)
		return;

	// free history
	Revision *hist = txt->history;
	while (hist && hist->prev)
		hist = hist->prev;
	while (hist) {
		Revision *later = hist->later;
		revision_free(hist);
		hist = later;
	}

	for (Piece *next, *p = txt->pieces; p; p = next) {
		next = p->global_next;
		piece_free(p);
	}

	for (size_t i = 0, len = array_length(&txt->blocks); i < len; i++)
		block_free(array_get_ptr(&txt->blocks, i));
	array_release(&txt->blocks);

	free(txt);
}

bool text_modified(const Text *txt) {
	return txt->saved_revision != txt->history;
}

bool text_mmaped(const Text *txt, const char *ptr) {
	uintptr_t addr = (uintptr_t)ptr;
	for (size_t i = 0, len = array_length(&txt->blocks); i < len; i++) {
		Block *blk = array_get_ptr(&txt->blocks, i);
		if ((blk->type == BLOCK_TYPE_MMAP_ORIG || blk->type == BLOCK_TYPE_MMAP) &&
		    (uintptr_t)(blk->data) <= addr && addr < (uintptr_t)(blk->data + blk->size))
			return true;
	}
	return false;
}

static bool iterator_init(Iterator *it, size_t pos, Piece *p, size_t off) {
	*it = (Iterator){
		.pos = pos,
		.piece = p,
		.start = p ? p->data : NULL,
		.end = p && p->data ? p->data + p->len : NULL,
		.text = p && p->data ? p->data + off : NULL,
	};
	return text_iterator_valid(it);
}

bool text_iterator_init(const Text *txt, Iterator *it, size_t pos) {
	Location loc = piece_get_extern(txt, pos);
	return iterator_init(it, pos, loc.piece, loc.off);
}

Iterator text_iterator_get(const Text *txt, size_t pos) {
	Iterator it;
	text_iterator_init(txt, &it, pos);
	return it;
}

bool text_iterator_next(Iterator *it) {
	size_t rem = it->end - it->text;
	return iterator_init(it, it->pos+rem, it->piece ? it->piece->next : NULL, 0);
}

bool text_iterator_prev(Iterator *it) {
	size_t off = it->text - it->start;
	size_t len = it->piece && it->piece->prev ? it->piece->prev->len : 0;
	return iterator_init(it, it->pos-off, it->piece ? it->piece->prev : NULL, len);
}

const Text *text_iterator_text(const Iterator *it) {
	return it->piece ? it->piece->text : NULL;
}

bool text_iterator_valid(const Iterator *it) {
	/* filter out sentinel nodes */
	return it->piece && it->piece->text;
}

bool text_iterator_has_next(const Iterator *it) {
	return it->piece && it->piece->next;
}

bool text_iterator_has_prev(const Iterator *it) {
	return it->piece && it->piece->prev;
}

size_t text_size(const Text *txt) {
	return txt->size;
}

/* count the number of new lines '\n' in range [pos, pos+len) */
static size_t lines_count(Text *txt, size_t pos, size_t len) {
	size_t lines = 0;
	for (Iterator it = text_iterator_get(txt, pos);
	     text_iterator_valid(&it);
	     text_iterator_next(&it)) {
		const char *start = it.text;
		while (len > 0 && start < it.end) {
			size_t n = MIN(len, (size_t)(it.end - start));
			const char *end = memchr(start, '\n', n);
			if (!end) {
				len -= n;
				break;
			}
			lines++;
			len -= end - start + 1;
			start = end + 1;
		}

		if (len == 0)
			break;
	}
	return lines;
}

/* skip n lines forward and return position afterwards */
static size_t lines_skip_forward(Text *txt, size_t pos, size_t lines, size_t *lines_skipped) {
	size_t lines_old = lines;
	for (Iterator it = text_iterator_get(txt, pos);
	     text_iterator_valid(&it);
	     text_iterator_next(&it)) {
		const char *start = it.text;
		while (lines > 0 && start < it.end) {
			size_t n = it.end - start;
			const char *end = memchr(start, '\n', n);
			if (!end) {
				pos += n;
				break;
			}
			pos += end - start + 1;
			start = end + 1;
			lines--;
		}

		if (lines == 0)
			break;
	}
	if (lines_skipped)
		*lines_skipped = lines_old - lines;
	return pos;
}

static void lineno_cache_invalidate(LineCache *cache) {
	cache->pos = 0;
	cache->lineno = 1;
}

size_t text_pos_by_lineno(Text *txt, size_t lineno) {
	size_t lines_skipped;
	LineCache *cache = &txt->lines;
	if (lineno <= 1)
		return 0;
	if (lineno > cache->lineno) {
		cache->pos = lines_skip_forward(txt, cache->pos, lineno - cache->lineno, &lines_skipped);
		cache->lineno += lines_skipped;
	} else if (lineno < cache->lineno) {
	#if 0
		// TODO does it make sense to scan memory backwards here?
		size_t diff = cache->lineno - lineno;
		if (diff < lineno) {
			lines_skip_backward(txt, cache->pos, diff);
		} else
	#endif
		cache->pos = lines_skip_forward(txt, 0, lineno - 1, &lines_skipped);
		cache->lineno = lines_skipped + 1;
	}
	return cache->lineno == lineno ? cache->pos : EPOS;
}

size_t text_lineno_by_pos(Text *txt, size_t pos) {
	LineCache *cache = &txt->lines;
	if (pos > txt->size)
		pos = txt->size;
	if (pos < cache->pos) {
		size_t diff = cache->pos - pos;
		if (diff < pos)
			cache->lineno -= lines_count(txt, pos, diff);
		else
			cache->lineno = lines_count(txt, 0, pos) + 1;
	} else if (pos > cache->pos) {
		cache->lineno += lines_count(txt, cache->pos, pos - cache->pos);
	}
	cache->pos = text_line_begin(txt, pos);
	return cache->lineno;
}

Mark text_mark_set(Text *txt, size_t pos) {
	if (pos == txt->size)
		return (Mark)&txt->end;
	Location loc = piece_get_extern(txt, pos);
	if (!loc.piece)
		return EMARK;
	return (Mark)(loc.piece->data + loc.off);
}

size_t text_mark_get(const Text *txt, Mark mark) {
	size_t cur = 0;

	if (mark == EMARK)
		return EPOS;
	if (mark == (Mark)&txt->end)
		return txt->size;

	for (Piece *p = txt->begin.next; p->next; p = p->next) {
		Mark start = (Mark)(p->data);
		Mark end = start + p->len;
		if (start <= mark && mark < end)
			return cur + (mark - start);
		cur += p->len;
	}

	return EPOS;
}