LuaJIT FFI 2D matrix with continuously allocated data array.

luajit_mat2d.c

#include <cinttypes>

extern "C" {
	__declspec(dllexport) void array_to_mat2d_byte(unsigned char* data, unsigned char** mat, size_t w, size_t h) {
		for (size_t i = 0; i < w; ++i) {
			mat[i] = data + h * i;
		}
	}

	__declspec(dllexport) void array_to_mat2d_int32(int32_t* data, int32_t** mat, size_t w, size_t h) {
		for (size_t i = 0; i < w; ++i) {
			mat[i] = data + h * i;
		}
	}

	__declspec(dllexport) void array_to_mat2d_int64(int64_t* data, int64_t** mat, size_t w, size_t h) {
		for (size_t i = 0; i < w; ++i) {
			mat[i] = data + h * i;
		}
	}

	__declspec(dllexport) void array_to_mat2d_float(float* data, float** mat, size_t w, size_t h) {
		for (size_t i = 0; i < w; ++i) {
			mat[i] = data + h * i;
		}
	}
}

luajit_mat2d.lua

ffi = require 'ffi'
local lib = ffi.load('mat2d')

ffi.cdef[[
    void array_to_mat2d_byte(unsigned char* data, unsigned char** mat, size_t w, size_t h);
    void array_to_mat2d_int32(int32_t* data, int32_t** mat, size_t w, size_t h);
    void array_to_mat2d_int64(int64_t* data, int64_t** mat, size_t w, size_t h);
    void array_to_mat2d_float(float* data, float** mat, size_t w, size_t h);
]]

local mat2d = {}

--[[
    the idea is `mat` is the short cut to access actual data in `data`.
    both reference must be kept to avoid gc.
--]]

function mat2d.byte(w, h)
    local data = ffi.new('unsigned char [?]', w * h)
    local mat = ffi.new('unsigned char *[?]', w)
    lib.array_to_mat2d_byte(data, mat, w, h)
    return data, mat
end

function mat2d.int32(w, h)
    local data = ffi.new('int32_t [?]', w * h)
    local mat = ffi.new('int32_t *[?]', w)
    lib.array_to_mat2d_int32(data, mat, w, h)
    return data, mat
end

function mat2d.int64(w, h)
    local data = ffi.new('int64_t [?]', w * h)
    local mat = ffi.new('int64_t *[?]', w)
    lib.array_to_mat2d_int64(data, mat, w, h)
    return data, mat
end

function mat2d.float(w, h)
    local data = ffi.new('float [?]', w * h)
    local mat = ffi.new('float *[?]', w)
    lib.array_to_mat2d_float(data, mat, w, h)
    return data, mat
end

return mat2d

luajit_mat2d_pure.lua

-- pure Lua version. A little slower than C version.

function mat2d.byte(w, h)
    local data = ffi.new('unsigned char [?]', w * h)
    local data_ptr = ffi.cast('unsigned char*', data)
    local mat = ffi.new('unsigned char *[?]', w)
    for i = 0, w - 1 do
        mat[i] = data_ptr + i * h
    end
    return data, mat
end