Understanding Transaction Input Counts

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Transaction-structure
Version
Marker & Flag
Prev Txid
...

Inputs: Input Count

The number of inputs is stored as a variable-length integer (varint).
Let's examine this real transaction:

Breaking this down:

01000000: Version (4 bytes)
01: Input count (1 byte, highlighted in the image below)
14e68f...: First input begins

As shown in the highlighted section of the transaction hex above, this transaction has an input count of 1 (0x01).
This single byte tells us that we should expect exactly one input in this transaction.

The encoding format depends on the value range:

Decimal Range	Hex Range	Bytes used	Format
0 to 252	0x00 to 0xfc	1	uint8_t
253 to 2¹⁶-1	0xfd to 0xffff	3	`0xfd` followed by the number as uint16_t
2¹⁶ to 2³²-1	0x10000 to 0xffffffff	5	`0xfe` followed by the number as uint32_t
2³² to 2⁶⁴-1	0x100000000 to 0xffffffffffffffff	9	`0xff` followed by the number as uint64_t

For example:

The number 100 is encoded directly as 0x64
The number 1000 is encoded as 0xfd 0xe8 0x03
The number 100000 is encoded as 0xfe 0xa0 0x86 0x01 0x00

ℹ️ Note: In our example transaction, the input count is 1 (0x01), so it uses the simplest encoding format of a single byte.

Implementation

Here's a Python implementation for reading and writing varints:


def read_varint(s):
    '''Reads a variable integer from a stream
    
    The first byte determines the format:
    - If < 0xfd: directly contains the number
    - If 0xfd: next 2 bytes contain number
    - If 0xfe: next 4 bytes contain number 
    - If 0xff: next 8 bytes contain number
    '''
    i = s.read(1)[0]
    if i == 0xfd:
        return int.from_bytes(s.read(2), 'little')
    elif i == 0xfe:
        return int.from_bytes(s.read(4), 'little')
    elif i == 0xff:
        return int.from_bytes(s.read(8), 'little')
    else:
        return i

def encode_varint(i):
'''Encodes an integer as a varint

    - Numbers < 0xfd: 1 byte
    - Numbers < 0x10000: 3 bytes (0xfd + 2 bytes)
    - Numbers < 0x100000000: 5 bytes (0xfe + 4 bytes)
    - Numbers < 0x10000000000000000: 9 bytes (0xff + 8 bytes)
    '''
    if i < 0xfd:
        return bytes([i])
    elif i < 0x10000:
        return b'\\xfd' + i.to_bytes(2, 'little')
    elif i < 0x100000000:
        return b'\\xfe' + i.to_bytes(4, 'little')
    elif i < 0x10000000000000000:
        return b'\\xff' + i.to_bytes(8, 'little')
    else:
        raise ValueError(f'integer too large: {i}')

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