feat: add cdecode module

* remove trailing tabs
* update cdecode.c to include "cdecode.h" more directly
  (to help with build on iOS)
* update README.md

README.md

@@ -1,7 +1,10 @@
-libb64: Base64 Encoding C Routine
-=================================
+libb64: Base64 C Routines
+=========================
 
-From project: <http://libb64.sourceforge.net/> (latest CVS source from <http://libb64.cvs.sourceforge.net/viewvc/libb64/>)
+From projects:
+
+- <https://github.com/gorb314/libb64>
+- <http://libb64.sourceforge.net/> (latest CVS source from <http://libb64.cvs.sourceforge.net/viewvc/libb64/>)
 
 LICENSE:
 --------
@@ -18,8 +21,8 @@ Major changes:
 --------------
 
 - Use macro instead of function to encode each value
-- Line breaks disabled by default
-- Include fixed for iOS build
+- Line breaks disabled in cencode.c by default
+- Flatter include statements to make iOS build easier
 
 Other versions:
 ---------------

cdecode.c

@@ -0,0 +1,88 @@
+/*
+cdecoder.c - c source to a base64 decoding algorithm implementation
+
+This is part of the libb64 project, and has been placed in the public domain.
+For details, see http://sourceforge.net/projects/libb64
+*/
+
+#include "cdecode.h"
+
+int base64_decode_value(char value_in)
+{
+	static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
+	static const char decoding_size = sizeof(decoding);
+	value_in -= 43;
+	if (value_in < 0 || value_in >= decoding_size) return -1;
+	return decoding[(int)value_in];
+}
+
+void base64_init_decodestate(base64_decodestate* state_in)
+{
+	state_in->step = step_a;
+	state_in->plainchar = 0;
+}
+
+int base64_decode_block(const char* code_in, const int length_in, char* plaintext_out, base64_decodestate* state_in)
+{
+	const char* codechar = code_in;
+	char* plainchar = plaintext_out;
+	char fragment;
+
+	*plainchar = state_in->plainchar;
+
+	switch (state_in->step)
+	{
+		while (1)
+		{
+	case step_a:
+			do {
+				if (codechar == code_in+length_in)
+				{
+					state_in->step = step_a;
+					state_in->plainchar = *plainchar;
+					return plainchar - plaintext_out;
+				}
+				fragment = (char)base64_decode_value(*codechar++);
+			} while (fragment < 0);
+			*plainchar    = (fragment & 0x03f) << 2;
+	case step_b:
+			do {
+				if (codechar == code_in+length_in)
+				{
+					state_in->step = step_b;
+					state_in->plainchar = *plainchar;
+					return plainchar - plaintext_out;
+				}
+				fragment = (char)base64_decode_value(*codechar++);
+			} while (fragment < 0);
+			*plainchar++ |= (fragment & 0x030) >> 4;
+			*plainchar    = (fragment & 0x00f) << 4;
+	case step_c:
+			do {
+				if (codechar == code_in+length_in)
+				{
+					state_in->step = step_c;
+					state_in->plainchar = *plainchar;
+					return plainchar - plaintext_out;
+				}
+				fragment = (char)base64_decode_value(*codechar++);
+			} while (fragment < 0);
+			*plainchar++ |= (fragment & 0x03c) >> 2;
+			*plainchar    = (fragment & 0x003) << 6;
+	case step_d:
+			do {
+				if (codechar == code_in+length_in)
+				{
+					state_in->step = step_d;
+					state_in->plainchar = *plainchar;
+					return plainchar - plaintext_out;
+				}
+				fragment = (char)base64_decode_value(*codechar++);
+			} while (fragment < 0);
+			*plainchar++   |= (fragment & 0x03f);
+		}
+	}
+	/* control should not reach here */
+	return plainchar - plaintext_out;
+}
+

cdecode.h

@@ -0,0 +1,29 @@
+/*
+cdecode.h - c header for a base64 decoding algorithm
+
+This is part of the libb64 project, and has been placed in the public domain.
+For details, see http://sourceforge.net/projects/libb64
+*/
+
+#ifndef BASE64_CDECODE_H
+#define BASE64_CDECODE_H
+
+typedef enum
+{
+	step_a, step_b, step_c, step_d
+} base64_decodestep;
+
+typedef struct
+{
+	base64_decodestep step;
+	char plainchar;
+} base64_decodestate;
+
+void base64_init_decodestate(base64_decodestate* state_in);
+
+int base64_decode_value(char value_in);
+
+int base64_decode_block(const char* code_in, const int length_in, char* plaintext_out, base64_decodestate* state_in);
+
+#endif /* BASE64_CDECODE_H */
+