#include <stdlib.h>

#include <unistd.h>

#include <string.h>

#include <lua.h>

#include <lauxlib.h>

typedef struct {

	size_t rpos; /* read position */

	size_t wpos; /* write position */

	size_t alen; /* allocated size */

	size_t blen; /* current content size */

	char buffer[];

} ringbuffer;

static void writechar(ringbuffer *b, char c) {

	b->blen++;

	b->buffer[(b->wpos++) % b->alen] = c;

}

/* make sure position counters stay within the allocation */

static void modpos(ringbuffer *b) {

	b->rpos = b->rpos % b->alen;

	b->wpos = b->wpos % b->alen;

}

static int find(ringbuffer *b, const char *s, size_t l) {

	size_t i, j;

	int m;

	if(b->rpos == b->wpos) { /* empty */

		return 0;

	}

	/* look for a matching first byte */

	for(i = 0; i <= b->blen - l; i++) {

		if(b->buffer[(b->rpos + i) % b->alen] == *s) {

			m = 1;

			/* check if the following byte also match */

			for(j = 1; j < l; j++)

				if(b->buffer[(b->rpos + i + j) % b->alen] != s[j]) {

					m = 0;

					break;

				}

			if(m) {

				return i + l;

			}

		}

	}

	return 0;

}

/*

 * Find first position of a substring in buffer

 * (buffer, string) -> number

 */

static int rb_find(lua_State *L) {

	size_t l, m;

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	const char *s = luaL_checklstring(L, 2, &l);

	m = find(b, s, l);

	if(m > 0) {

		lua_pushinteger(L, m);

		return 1;

	}

	return 0;

}

/*

 * Move read position forward without returning the data

 * (buffer, number) -> boolean

 */

static int rb_discard(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	size_t r = luaL_checkinteger(L, 2);

	if(r > b->blen) {

		lua_pushboolean(L, 0);

		return 1;

	}

	b->blen -= r;

	b->rpos += r;

	modpos(b);

	lua_pushboolean(L, 1);

	return 1;

}

/*

 * Read bytes from buffer

 * (buffer, number, boolean?) -> string

 */

static int rb_read(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	size_t r = luaL_checkinteger(L, 2);

	int peek = lua_toboolean(L, 3);

	if(r > b->blen) {

		lua_pushnil(L);

		return 1;

	}

	if((b->rpos + r) > b->alen) {

		/* Substring wraps around to the beginning of the buffer */

		lua_pushlstring(L, &b->buffer[b->rpos], b->alen - b->rpos);

		lua_pushlstring(L, b->buffer, r - (b->alen - b->rpos));

		lua_concat(L, 2);

	} else {

		lua_pushlstring(L, &b->buffer[b->rpos], r);

	}

	if(!peek) {

		b->blen -= r;

		b->rpos += r;

		modpos(b);

	}

	return 1;

}

/*

 * Read buffer until first occurrence of a substring

 * (buffer, string) -> string

 */

static int rb_readuntil(lua_State *L) {

	size_t l, m;

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	const char *s = luaL_checklstring(L, 2, &l);

	m = find(b, s, l);

	if(m > 0) {

		lua_settop(L, 1);

		lua_pushinteger(L, m);

		return rb_read(L);

	}

	return 0;

}

/*

 * Write bytes into the buffer

 * (buffer, string) -> integer

 */

static int rb_write(lua_State *L) {

	size_t l, w = 0;

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	const char *s = luaL_checklstring(L, 2, &l);

	/* Does `l` bytes fit? */

	if((l + b->blen) > b->alen) {

		lua_pushnil(L);

		return 1;

	}

	while(l-- > 0) {

		writechar(b, *s++);

		w++;

	}

	modpos(b);

	lua_pushinteger(L, w);

	return 1;

}

static int rb_tostring(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	lua_pushfstring(L, "ringbuffer: %p %d/%d", b, b->blen, b->alen);

	return 1;

}

static int rb_length(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	lua_pushinteger(L, b->blen);

	return 1;

}

static int rb_size(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	lua_pushinteger(L, b->alen);

	return 1;

}

static int rb_free(lua_State *L) {

	ringbuffer *b = luaL_checkudata(L, 1, "ringbuffer_mt");

	lua_pushinteger(L, b->alen - b->blen);

	return 1;

}

static int rb_new(lua_State *L) {

	lua_Integer size = luaL_optinteger(L, 1, sysconf(_SC_PAGESIZE));

	luaL_argcheck(L, size > 0, 1, "positive integer expected");

	ringbuffer *b = lua_newuserdata(L, sizeof(ringbuffer) + size);

	b->rpos = 0;

	b->wpos = 0;

	b->alen = size;

	b->blen = 0;

	luaL_getmetatable(L, "ringbuffer_mt");

	lua_setmetatable(L, -2);

	return 1;

}

int luaopen_util_ringbuffer(lua_State *L) {

#if (LUA_VERSION_NUM > 501)

	luaL_checkversion(L);

#endif

	if(luaL_newmetatable(L, "ringbuffer_mt")) {

		lua_pushcfunction(L, rb_tostring);

		lua_setfield(L, -2, "__tostring");

		lua_pushcfunction(L, rb_length);

		lua_setfield(L, -2, "__len");

		lua_createtable(L, 0, 7); /* __index */

		{

			lua_pushcfunction(L, rb_find);

			lua_setfield(L, -2, "find");

			lua_pushcfunction(L, rb_discard);

			lua_setfield(L, -2, "discard");

			lua_pushcfunction(L, rb_read);

			lua_setfield(L, -2, "read");

			lua_pushcfunction(L, rb_readuntil);

			lua_setfield(L, -2, "readuntil");

			lua_pushcfunction(L, rb_write);

			lua_setfield(L, -2, "write");

			lua_pushcfunction(L, rb_size);

			lua_setfield(L, -2, "size");

			lua_pushcfunction(L, rb_length);

			lua_setfield(L, -2, "length");

			lua_pushcfunction(L, rb_free);

			lua_setfield(L, -2, "free");

		}

		lua_setfield(L, -2, "__index");

	}

	lua_createtable(L, 0, 1);

	lua_pushcfunction(L, rb_new);

	lua_setfield(L, -2, "new");

	return 1;

}