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/*
* A very simple font cache and rasterizer that uses freetype
* to draw fonts from a single OpenGL texture. The code uses
* a linear-probe hashtable, and writes new glyphs into
* the texture using glTexSubImage2D. When the texture fills
* up, or the hash table gets too crowded, everything is wiped.
*
* This is designed to be used for horizontal text only,
* and draws unhinted text with subpixel accurate metrics
* and kerning. As such, you should always call the drawing
* function with an identity transform that maps units
* to pixels accurately.
*
* If you wish to use it to draw arbitrarily transformed
* text, change the min and mag filters to GL_LINEAR and
* add a pixel of padding between glyphs and rows, and
* make sure to clear the texture when wiping the cache.
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "opengl.h"
#include "utf.h"

#include <ft2build.h>
#include FT_FREETYPE_H
#include FT_ADVANCES_H

#define PADDING 1 /* set to 0 to save some space but disallow arbitrary transforms */

#define MAXGLYPHS 4093 /* prime number for hash table goodness */
#define CACHESIZE 256
#define XPRECISION 4
#define YPRECISION 1

static inline void die(char *msg)
{
fprintf(stderr, "error: %s\n", msg);
exit(1);
}

struct key
{
FT_Face face;
short size;
short gid;
short subx;
short suby;
};

struct glyph
{
char lsb, top, w, h;
short s, t;
float advance;
};

struct table
{
struct key key;
struct glyph glyph;
};

static FT_Library g_freetype_lib = NULL;
static struct table g_table[MAXGLYPHS];
static int g_table_load = 0;
static unsigned int g_cache_tex = 0;
static int g_cache_w = CACHESIZE;
static int g_cache_h = CACHESIZE;
static int g_cache_row_y = 0;
static int g_cache_row_x = 0;
static int g_cache_row_h = 0;

static void init_font_cache(void)
{
int code;

code = FT_Init_FreeType(&g_freetype_lib);
if (code)
die("cannot initialize freetype");

glGenTextures(1, &g_cache_tex);
glBindTexture(GL_TEXTURE_2D, g_cache_tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, g_cache_w, g_cache_h, 0, GL_ALPHA, GL_UNSIGNED_BYTE, NULL);
}

static void clear_font_cache(void)
{
#if PADDING > 0
unsigned char *zero = malloc(g_cache_w * g_cache_h);
memset(zero, 0, g_cache_w * g_cache_h);
glBindTexture(GL_TEXTURE_2D, g_cache_tex);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, g_cache_w, g_cache_h, GL_ALPHA, GL_UNSIGNED_BYTE, zero);
free(zero);
#endif

memset(g_table, 0, sizeof(g_table));
g_table_load = 0;

g_cache_row_y = PADDING;
g_cache_row_x = PADDING;
g_cache_row_h = 0;
}

FT_Face load_font(char *fontname)
{
FT_Face face;
int code;

if (g_freetype_lib == NULL)
{
init_font_cache();
clear_font_cache();
}

code = FT_New_Face(g_freetype_lib, fontname, 0, &face);
if (code)
die("cannot load font file");

FT_Select_Charmap(face, ft_encoding_unicode);

return face;
}

void free_font(FT_Face face)
{
clear_font_cache();
FT_Done_Face(face);
}

static unsigned int hashfunc(struct key *key)
{
unsigned char *buf = (unsigned char *)key;
unsigned int len = sizeof(struct key);
unsigned int h = 0;
while (len--)
h = *buf++ + (h << 6) + (h << 16) - h;
return h;
}

static unsigned int lookup_table(struct key *key)
{
unsigned int pos = hashfunc(key) % MAXGLYPHS;
while (1)
{
if (!g_table[pos].key.face) /* empty slot */
return pos;
if (!memcmp(key, &g_table[pos].key, sizeof(struct key))) /* matching slot */
return pos;
pos = (pos + 1) % MAXGLYPHS;
}
}

static struct glyph * lookup_glyph(FT_Face face, int size, int gid, int subx, int suby)
{
FT_Vector subv;
struct key key;
unsigned int pos;
int code;
int w, h;

/*
* Look it up in the table
*/

key.face = face;
key.size = size;
key.gid = gid;
key.subx = subx;
key.suby = suby;

pos = lookup_table(&key);
if (g_table[pos].key.face)
return &g_table[pos].glyph;

/*
* Render the bitmap
*/

glEnd();

subv.x = subx;
subv.y = suby;

FT_Set_Transform(face, NULL, &subv);

code = FT_Load_Glyph(face, gid, FT_LOAD_NO_BITMAP | FT_LOAD_NO_HINTING);
if (code < 0)
return NULL;

code = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_LIGHT);
if (code < 0)
return NULL;

w = face->glyph->bitmap.width;
h = face->glyph->bitmap.rows;

/*
* Find an empty slot in the texture
*/

if (g_table_load == (MAXGLYPHS * 3) / 4)
{
puts("font cache table full, clearing cache");
clear_font_cache();
pos = lookup_table(&key);
}

if (h + PADDING > g_cache_h || w + PADDING > g_cache_w)
die("rendered glyph exceeds cache dimensions");

if (g_cache_row_x + w + PADDING > g_cache_w)
{
g_cache_row_y += g_cache_row_h + PADDING;
g_cache_row_x = PADDING;
g_cache_row_h = 0;
}
if (g_cache_row_y + h + PADDING > g_cache_h)
{
puts("font cache texture full, clearing cache");
clear_font_cache();
pos = lookup_table(&key);
}

/*
* Copy bitmap into texture
*/

memcpy(&g_table[pos].key, &key, sizeof(struct key));
g_table[pos].glyph.w = face->glyph->bitmap.width;
g_table[pos].glyph.h = face->glyph->bitmap.rows;
g_table[pos].glyph.lsb = face->glyph->bitmap_left;
g_table[pos].glyph.top = face->glyph->bitmap_top;
g_table[pos].glyph.s = g_cache_row_x;
g_table[pos].glyph.t = g_cache_row_y;
g_table[pos].glyph.advance = face->glyph->advance.x / 64.0;
g_table_load ++;

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glPixelStorei(GL_UNPACK_ROW_LENGTH, face->glyph->bitmap.pitch);
glTexSubImage2D(GL_TEXTURE_2D, 0, g_cache_row_x, g_cache_row_y, w, h,
GL_ALPHA, GL_UNSIGNED_BYTE, face->glyph->bitmap.buffer);
glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);

glBegin(GL_QUADS);

g_cache_row_x += w + PADDING;
if (g_cache_row_h < h + PADDING)
g_cache_row_h = h + PADDING;

return &g_table[pos].glyph;
}

static float draw_glyph(FT_Face face, int size, int gid, float x, float y)
{
struct glyph *glyph;
int subx = (x - floor(x)) * XPRECISION;
int suby = (y - floor(y)) * YPRECISION;
subx = (subx * 64) / XPRECISION;
suby = (suby * 64) / YPRECISION;

glyph = lookup_glyph(face, size, gid, subx, suby);
if (!glyph)
return 0.0;

float s0 = (float) glyph->s / g_cache_w;
float t0 = (float) glyph->t / g_cache_h;
float s1 = (float) (glyph->s + glyph->w) / g_cache_w;
float t1 = (float) (glyph->t + glyph->h) / g_cache_h;
float xc = floor(x) + glyph->lsb;
float yc = floor(y) - glyph->top + glyph->h;

glTexCoord2f(s0, t0); glVertex2f(xc, yc - glyph->h);
glTexCoord2f(s1, t0); glVertex2f(xc + glyph->w, yc - glyph->h);
glTexCoord2f(s1, t1); glVertex2f(xc + glyph->w, yc);
glTexCoord2f(s0, t1); glVertex2f(xc, yc);

return glyph->advance;
}

float measure_string(FT_Face face, float fsize, char *str)
{
int size = fsize * 64;
FT_Fixed advance;
FT_Vector kern;
Rune ucs, gid;
float w = 0.0;
int left = 0;

FT_Set_Char_Size(face, size, size, 72, 72);

while (*str)
{
str += chartorune(&ucs, str);
gid = FT_Get_Char_Index(face, ucs);
FT_Get_Kerning(face, left, gid, FT_KERNING_UNFITTED, &kern);
w += kern.x / 64.0;
FT_Get_Advance(face, gid, FT_LOAD_NO_BITMAP | FT_LOAD_NO_HINTING, &advance);
w += advance / 65536.0;
left = gid;
}

return w;
}

float draw_string(FT_Face face, float fsize, float x, float y, char *str)
{
int size = fsize * 64;
FT_Vector kern;
Rune ucs, gid;
int left = 0;

FT_Set_Char_Size(face, size, size, 72, 72);

glBindTexture(GL_TEXTURE_2D, g_cache_tex);
glBegin(GL_QUADS);

while (*str)
{
str += chartorune(&ucs, str);
gid = FT_Get_Char_Index(face, ucs);
FT_Get_Kerning(face, left, gid, FT_KERNING_UNFITTED, &kern);
x += kern.x / 64.0;
x += draw_glyph(face, size, gid, x, y);
left = gid;
}

glEnd();

return x;
}

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