core/common: Add BytearrayReader and basic reader functions.

core/src/apps/bitcoin/readers.py

@@ -0,0 +1,25 @@
+from apps.common.readers import BytearrayReader, read_bitcoin_varint
+
+
+def read_bytes_prefixed(r: BytearrayReader) -> bytes:
+    n = read_bitcoin_varint(r)
+    return r.read(n)
+
+
+def read_op_push(r: BytearrayReader) -> int:
+    prefix = r.get()
+    if prefix < 0x4C:
+        n = prefix
+    elif prefix == 0x4C:
+        n = r.get()
+    elif prefix == 0x4D:
+        n = r.get()
+        n += r.get() << 8
+    elif prefix == 0x4E:
+        n = r.get()
+        n += r.get() << 8
+        n += r.get() << 16
+        n += r.get() << 24
+    else:
+        raise ValueError
+    return n

core/src/apps/common/readers.py

@@ -0,0 +1,47 @@
+if False:
+    from typing import Optional
+
+
+class BytearrayReader:
+    def __init__(self, data: bytes):
+        self.data = data
+        self.offset = 0
+
+    def get(self) -> int:
+        ret = self.data[self.offset]
+        self.offset += 1
+        return ret
+
+    def peek(self) -> int:
+        return self.data[self.offset]
+
+    def read(self, i: Optional[int] = None) -> bytes:
+        if i is None:
+            ret = self.data[self.offset :]
+            self.offset = len(self.data)
+        elif 0 <= i <= len(self.data) - self.offset:
+            ret = self.data[self.offset : self.offset + i]
+            self.offset += i
+        else:
+            raise IndexError
+        return ret
+
+    def remaining_count(self) -> int:
+        return len(self.data) - self.offset
+
+
+def read_bitcoin_varint(r: BytearrayReader) -> int:
+    prefix = r.get()
+    if prefix < 253:
+        n = prefix
+    elif prefix == 253:
+        n = r.get()
+        n += r.get() << 8
+    elif prefix == 254:
+        n = r.get()
+        n += r.get() << 8
+        n += r.get() << 16
+        n += r.get() << 24
+    else:
+        raise ValueError
+    return n