implement adpcm encoding and make frame speed variable

CMakeLists.txt

@@ -3,6 +3,7 @@ cmake_minimum_required(VERSION 3.8.0 FATAL_ERROR)
 project(dsiflipencode)
 
 set(SOURCES
+	src/adpcm_encoder.cpp
     src/lodepng.cpp
     src/main.cpp
     src/util.cpp

include/adpcm_encoder.h

@@ -0,0 +1,28 @@
+#pragma once
+// implementation is from libavcodec
+#include <cstdint>
+
+constexpr int8_t ff_adpcm_index_table[16] = {
+    -1, -1, -1, -1, 2, 4, 6, 8,
+    -1, -1, -1, -1, 2, 4, 6, 8,
+};
+
+constexpr int16_t ff_adpcm_step_table[89] = {
+        7,     8,     9,    10,    11,    12,    13,    14,    16,    17,
+       19,    21,    23,    25,    28,    31,    34,    37,    41,    45,
+       50,    55,    60,    66,    73,    80,    88,    97,   107,   118,
+      130,   143,   157,   173,   190,   209,   230,   253,   279,   307,
+      337,   371,   408,   449,   494,   544,   598,   658,   724,   796,
+      876,   963,  1060,  1166,  1282,  1411,  1552,  1707,  1878,  2066,
+     2272,  2499,  2749,  3024,  3327,  3660,  4026,  4428,  4871,  5358,
+     5894,  6484,  7132,  7845,  8630,  9493, 10442, 11487, 12635, 13899,
+    15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
+};
+
+class AdpcmEncoder {
+public:
+	int16_t prev_sample = 0;
+	int step_index = 0;
+
+	uint8_t EncodeSample(int16_t sample);
+};