1 files changed, 56 insertions, 0 deletions
diff --git a/src/bitfield/8byte.c b/src/bitfield/8byte.c
new file mode 100644
index 00000000..0f249d93
--- /dev/null
+++ b/src/bitfield/8byte.c
@@ -0,0 +1,56 @@
+#include <bitfield/bitfield.h>
+#include <bitfield/8byte.h>
+#include <stddef.h>
+#include <limits.h>
+#include <string.h>
+
+uint64_t bitmask(const uint8_t numBits) {
+    return (((uint64_t)0x1) << numBits) - 1;
+}
+
+static uint16_t bitsToBytes(uint32_t bits) {
+    uint8_t byte_count = bits / CHAR_BIT;
+    if(bits % CHAR_BIT != 0) {
+        ++byte_count;
+    }
+    return byte_count;
+}
+
+uint64_t getBitField(uint64_t data, const uint16_t startBit,
+        const uint16_t numBits, bool bigEndian) {
+    uint8_t result[8] = {0};
+    if(!bigEndian) {
+        data = __builtin_bswap64(data);
+    }
+    copyBitsRightAligned((const uint8_t*)&data, sizeof(data), startBit, numBits,
+            result, sizeof(result));
+    uint64_t int_result = 0;
+
+    if(!bigEndian) {
+        // we need to swap the byte order of the array to get it into a
+        // uint64_t, but it's been right aligned so we have to be more careful
+        for(int i = 0; i < bitsToBytes(numBits); i++) {
+            int_result |= result[bitsToBytes(numBits) - i - 1] << (CHAR_BIT * i);
+        }
+    } else {
+        int_result = *(uint64_t*)result;
+    }
+    return int_result;
+}
+
+/**
+ * TODO it would be nice to have a warning if you call with this a value that
+ * won't fit in the number of bits you've specified it should use.
+ */
+void setBitField(uint64_t* data, uint64_t value, const uint16_t startPos,
+        const uint16_t numBits) {
+    int shiftDistance = 64 - startPos - numBits;
+    value <<= shiftDistance;
+    *data &= ~(bitmask(numBits) << shiftDistance);
+    *data |= value;
+}
+
+uint8_t nthByte(const uint64_t source, const uint16_t byteNum) {
+    return (source >> (64 - ((byteNum + 1) * CHAR_BIT))) & 0xFF;
+}
+