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i think i figured out the boundary case crap

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pharpend
2023-05-15 16:57:02 -06:00
parent 93ce93a3bf
commit 60f56cf25d
1 changed files with 67 additions and 42 deletions
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libs/vdk/src/bin.ts
+67 -42
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@@ -258,7 +258,7 @@ bits_concat
// 0123_4567 8
// this is the bit index of the leftmost bit in this byte
let start_bit_bi0 : number = i * 8;
let next_start_bit_bi0 : number = first_bit_bi0 + 8;
let next_start_bit_bi0 : number = start_bit_bi0 + 8;
// does the bit at the beginning of this byte correspond to the first array?
// strict comparison:
// suppose i = 0,
@@ -277,6 +277,63 @@ bits_concat
// is this a bytes1 byte
let is_bytes1_byte : boolean = start_bit_is_of_first_array && stop_bit_is_of_first_array;
let is_boundary_byte : boolean = start_bit_is_of_first_array && !stop_bit_is_of_first_array;
// need to work out the ping_pong bs up here because js is dumb and I
// can't put lets between elseifs
// alright, now we're in the case of only copying from the second array
// we have two cases:
// ping-pong:
// ABCD_EFGH 1234_5678
// - ---- ---
// copying these bits
// ping:
// ABCD_EFGH <end>
// - ---- 000
//
// how to distinguish between these two??
//
// we're in the ping case when the start_bit_bi0 corresponds to the
// final byte of the second array
//
// ok
//
// we need to compute the bit address in the second array that
// corresponds to the bit address at the beginning of this byte in the
// result array
let bits2_addr_bi0 : number = start_bit_bi0 - bits1.bit_length;
// I think the variable is
// bits_left_to_copy = bits2.bit_length - bit_addr2_bi0
// no + 1 because the current bit is uncopied,
// so if bit_addr2_bi0 = 7 and bits2.bit_length is 8, it means we
// have the last bit to copy
// cases:
// bits_left_to_copy <= 0 ->
// this would mean we have more bits to copy, but are out of
// source bits. should be impossible if bits_null is correct
// bits_left_to_copy <= 8 ->
// this would mean we are going to fill this last byte in the
// result array with the correct bits from array2, but how we
// do this will depend on how those are arranged in bytes2
// (whether we grab one or two bytes)
//
// this is the tricky case
//
// I think what matters here is the byte address in the second array
// if we're on the last byte
// 8 < bits_left_to_copy ->
// this means we can safely grab two bytes from bytes2, and do
// our bitshifting to make it correct
//
// FIXME
let bits_left_to_copy : number = bits2.bit_length - bits2_addr_bi0;
// no the variable that matters is which byte we're on in the result
let this_bytes2_addr_i0 : number = Math.floor(bits2_addr_bi0 / 8);
let next_bytes2_addr_i0 : number = this_bytes2_addr_i0 + 1;
let bitshift_amt : number = bits1.bit_length % 8;
let ping : boolean = next_bytes2_addr_i0 === bytes2.length;
// simple case: this is a byte from the first array
// just copy it
if (is_bytes1_byte)
@@ -306,47 +363,15 @@ bits_concat
let bitshift_amt : number = bits1.bit_length % 8;
result_bytes[i] ^= first_byte_of_second_array >> bitshift_amt;
}
// alright, now we're in the case of only copying from the second array
// we have two cases:
// ping-pong:
// ABCD_EFGH 1234_5678
// - ---- ---
// copying these bits
// ping:
// ABCD_EFGH <end>
// - ---- 000
//
// how to distinguish between these two??
//
// we're in the ping case when the start_bit_bi0 corresponds to the
// final byte of the second array
//
// ok
//
// we need to compute the bit address in the second array that
// corresponds to the bit address at the beginning of this byte in the
// result array
let bit_addr2_bi0 = start_bit_bi0 - bits1.bit_length;
// I think the variable is
// bits_left_to_copy = bits2.bit_length - bit_addr2_bi0
// cases:
// bits_left_to_copy <= 0 ->
// this would mean we have more bits to copy, but are out of
// source bits. should be impossible if bits_null is correct
// bits_left_to_copy <= 8 ->
// this would mean we are going to fill this last byte in the
// result array with the correct bits from array2, but how we
// do this will depend on how those are arranged in bytes2
// (whether we grab one or two bytes)
//
// this is the tricky case
// 8 < bits_left_to_copy ->
// this means we can safely grab two bytes from bytes2, and do
// our bitshifting to make it correct
//
// FIXME
let ping_pong : boolean
// last byte of second array
else if (ping)
result_bytes[i] = (bytes2[this_bytes2_addr_i0] << bitshift_amt);
// ping-pong: not last byte of second array
else
result_bytes[i] = (bytes2[this_bytes2_addr_i0] << bitshift_amt) ^ (bytes2[next_bytes2_addr_i0] << bitshift_amt);
}
return {bit_length : result_bit_length,
bytes : result_bytes};
}