From 5eec31a61fe444cfd7b70fa90acfb41cb5e07f18 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Sat, 28 Jul 2018 21:05:26 +0200
Subject: [PATCH] need CRCs way to often

---
 README.md | 16 ++++++++++++
 crc.hh    | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)
 create mode 100644 crc.hh

diff --git a/README.md b/README.md
index 38342df..960cf16 100644
--- a/README.md
+++ b/README.md
@@ -38,3 +38,19 @@ Read and write [WAV](https://en.wikipedia.org/wiki/WAV) files
 Algorithm for computing uniform and [natural cubic splines](https://en.wikipedia.org/wiki/Spline_(mathematics)#Algorithm_for_computing_natural_cubic_splines)
 Very useful for data interpolation.
 
+### [crc.hh](crc.hh)
+
+A [Cyclic redundancy check](https://en.wikipedia.org/wiki/Cyclic_redundancy_check) may not really be DSP-related, but it is needed over and over again when you do DSP that it fits just perfectly here.
+
+For example, if we need to integrate CRC32 checking for a few bytes, like in the following:
+```
+# echo -n 'Hello World!' | rhash -C -
+(stdin) 1C291CA3
+```
+We can add it to our project as simple as that:
+```
+DSP::CRC<uint32_t> crc(0xEDB88320, 0xFFFFFFFF);
+for (uint8_t c: std::string("Hello World!")) crc(c);
+assert(!crc(uint32_t(~0x1C291CA3)));
+```
+
diff --git a/crc.hh b/crc.hh
new file mode 100644
index 0000000..3ff8ede
--- /dev/null
+++ b/crc.hh
@@ -0,0 +1,77 @@
+/*
+Cyclic redundancy check
+
+Copyright 2018 Ahmet Inan <inan@aicodix.de>
+*/
+
+#ifndef CRC_HH
+#define CRC_HH
+
+namespace DSP {
+
+template <typename TYPE>
+class CRC
+{
+	static const int BYTES = sizeof(TYPE);
+	TYPE lut[BYTES * 256];
+	TYPE poly;
+	TYPE crc;
+public:
+	CRC(TYPE poly, TYPE crc = 0) : poly(poly), crc(crc)
+	{
+		// exploit linearity of CRC only bytewise
+		for (int k = 0; k < BYTES; ++k) {
+			for (int j = 0; j < 256; ++j) {
+				TYPE tmp = j << (k << 3);
+				for (int i = 8; i; --i)
+					tmp = (tmp >> 1) ^ ((tmp & 1) * poly);
+				lut[(k << 8) + j] = tmp;
+			}
+		}
+	}
+	void reset(TYPE v)
+	{
+		crc = v;
+	}
+	TYPE operator()(bool data)
+	{
+		TYPE tmp = crc ^ data;
+		tmp = (tmp >> 1) ^ ((tmp & 1) * poly);
+		return crc = tmp;
+	}
+	TYPE operator()(uint8_t data)
+	{
+		TYPE tmp = crc ^ data;
+		crc = lut[tmp & 255];
+		for (int k = 1; k < BYTES; ++k)
+			crc ^= lut[(k << 8) + ((tmp >>= 8) & 255)];
+		return crc;
+	}
+	template <typename DATA>
+	TYPE operator()(DATA data)
+	{
+		for (int i = sizeof(data); i; --i) {
+			(*this)((uint8_t)data);
+			data >>= 8;
+		}
+		return crc;
+	}
+};
+
+template<>
+uint16_t CRC<uint16_t>::operator()(uint8_t data)
+{
+	uint16_t tmp = crc ^ data;
+	return crc = lut[256 + (tmp >> 8)] ^ lut[tmp & 255];
+}
+
+template<>
+uint8_t CRC<uint8_t>::operator()(uint8_t data)
+{
+	return crc = lut[crc ^ data];
+}
+
+}
+
+#endif
+