#!/bin/bash -eu

BITCOIND="bitcoind"
#BITCOIND="bitcoind -testnet"

if [ $# -ne 1 ]
then
    echo "Command to change the locktime of a bitcoin transaction provided on standard input"
    echo "  Usage: ... | $0 '<date>'"
    echo "  Example: btc-spend-all | $0 'tomorrow'"
    exit 1
fi

## I usually use this script in a pipeline with btc-spend-all, i.e.:
##    btc-spend-all | btc-change-locktime tomorrow | bitcoind signrawtransaction $(cat)
## or
##    echo '01000000017f286f35362457e961925835bff28351e0b679c7810827396c76967712a6714d0000000000ffffffff0100e1f505000000001976a9144c56a8926eeb911b06575be05bad06e66463fb1e88ac00000000' | btc-change-locktime
## Load the raw transaction from standard in
RAW_TRANSACTION=$(cat)

## Get the desired locktime in human-readable format and convert it into
## epoch time in both decimal format and little-endian hexadecimal format
LOCKTIME_DECIMAL=$( date +%s -ud "$1" ) || { echo 'Invalid date' ; exit 1 ; }
LOCKTIME_HEX=$( echo $LOCKTIME_DECIMAL | awk '{ printf "%x", $1 }' | sed 's/^\(..\)\(..\)\(..\)\(..\)/\4\3\2\1/' )


#### SAMPLE TRANSACTION
# 01000000017f286f35362457e961925835bff28351e0b679c7810827396c76967712a6714d0000000000ffffffff0100e1f505000000001976a9144c56a8926eeb911b06575be05bad06e66463fb1e88ac00000000
#### Every two characters (char) in hex equals one byte
# 01000000 .............................Version (8 char)
# 01 ...................................Vin count (2 char)
# 7f286f35362457e961925835bff28351e0b679c7810827396c76967712a6714d ...Prevout hash (64 char)
# 00000000 .............................Prevout index (8 char)
# 00 ...................................Signature script length (2 char)
# ffffffff .............................**Sequence number** (8 char)
# ++++++++++++++++++++++++++++++++++++++[[[[ additional inputs can go here ]]]]
# 01 ...................................vout count (2 char)
# 00e1f50500000000 .....................satoshis to transfer to send in this output (16 char)
# 19 ...................................public key length (2 char)
# 76a9144c56a8926eeb911b06575be05bad06e66463fb1e88ac public key claim script (50 char)
# ++++++++++++++++++++++++++++++++++++++[[[[ additional outputs can go here ]]]]
# 00000000 .............................**locktime** (8 char)

## Modify the transaction
MODIFIED_TRANSACTION=$( 
    echo $RAW_TRANSACTION | sed '''
        ## createrawtransaction by default creates transactions with
        ## max-value sequence numbers, which prevents locktime from having
        ## any effect. We need to change one of these sequence numbers to a
        ## lower value. Since the first sequence number always appears in
        ## the same byte location within the encoded transaction (after 84
        ## hexits), we will reduce it down to zero.
        s/\(.\{84\}\)ffffffff/\100000000/;

        ## Now we need to set locktime to the earliest date when we want to
        ## be able to spend the bitcoins. Locktime is always the last 8
        ## hexits of the encode transaction, and by default it is set to zero.
        s/00000000$/'$LOCKTIME_HEX'/;
    '''
)

## Pre-build the decode command so we can reuse it several times
_dc_cmd="$BITCOIND decoderawtransaction"

## Now lets diff the original and modified transactions to test for changes.
## The diff should always produce 11 lines of output: a two-line header
## listing the file names, three one-line headers for each changed part,
## and three two-line diffs
_diff_line_count=$( eval diff -u0 <( $_dc_cmd $RAW_TRANSACTION ) <( $_dc_cmd $MODIFIED_TRANSACTION ) | wc -l )
if [ $_diff_line_count -ne 11 ]
then
    echo Unexpected output from the diff command.  Dying...
    exit 1
fi

## Now test that all of the expected changes were made
eval diff -u0 <( $_dc_cmd $RAW_TRANSACTION ) <( $_dc_cmd $MODIFIED_TRANSACTION ) \
        | egrep -v  '^(---|\+\+\+|@@) '  \
        | sed '''
            ## Whitelist all of the expected changes in the diff by
            ## deleting them so we can then test for unexpected changes
            #
            ## The new locktime should decode to the decimal locktime
            /^+.*"locktime".*'$LOCKTIME_DECIMAL'/d;
            ## The old locktime should have been zero
            /^-.*"locktime".* 0,/d;
            ## The new sequence should be zero
            /^+.*"sequence".* 0$/d;
            ## The old sequence should have been the maximum, 4294967295
            /^-.*"sequence".* 4294967295/d;
            ## The only thing left should be the automatically-computerd
            ## txid hashes
            /"txid" : /d

            ## If anything remains, somenthing went wrong
        ''' | grep . && { echo "Unexpected changes.  Dying..." ; exit 1 ; }

## If we get here, all of the tests have been passed, so give the user
## the modified transaction so they can sign it
echo $MODIFIED_TRANSACTION