0.9.11-incubating/libguac/hash_8h_source.html

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 #ifndef _GUAC_HASH_H
 #define _GUAC_HASH_H

 #include <cairo/cairo.h>

 unsigned int guac_hash_surface(cairo_surface_t* surface);

 int guac_surface_cmp(cairo_surface_t* a, cairo_surface_t* b);

 #endif

guac_surface_cmp
int guac_surface_cmp(cairo_surface_t *a, cairo_surface_t *b)
Given two Cairo surfaces, returns zero if the data contained within each is identical, and a positive or negative value if the value of the first is found to be lexically greater or less than the second respectively.

guac_hash_surface
unsigned int guac_hash_surface(cairo_surface_t *surface)
Produces a 24-bit hash value from all pixels of the given surface.