new text compression and tokeniser - faster, about 100 bytes saved
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// #LAYOUT# STD * #TAKE
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// #LAYOUT# M65 BASIC_1 #TAKE
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// #LAYOUT# X16 BASIC_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Packs a keyword candidate using a simple compression scheme,
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// same as in 'generate_strings.cc'.
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//
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// This allows us to search through the compressed tokens directly
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// using the compressed fragment of BASIC, and thus work out the
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// token for a given piece of string.
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//
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// This also helps ensure that we have an implementation that is totally
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// different to that of the original C64 BASIC, as well as likely
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// saving a bit of space in the ROM, especially once we start implementing
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// larger keyword vocabs for extended BASIC, since the compression and
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// decompression machinery is of fixed size.
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//
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// Input:
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// - .X = offset of string to pack, address is BUF ($0200) + .X
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// Output:
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// - see labels in 'tk_pack'
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//
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tk_pack:
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// Reuse the CPU stack - addresses below $102 are used by 'tokenise_line.s'
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.label tk__len_unpacked = $103 // length of unpacked data; it could replace 'tk__nibble_flag', but at the cost of code size/performance
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.label tk__shorten_bits = $104 // for quick shortening of packed candidate
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.label tk__nibble_flag = $105 // $00 = start from new byte, $FF = start from high nibble
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.label tk__byte_offset = $106 // offset of the current byte (to place new data) in tk__packed
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.label tk__packed = $107 // packed candidate, 13 bytes is enough for worst case - 8 byte keyword
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// Initialize variables
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lda #$00
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ldy #(tk__packed - tk__len_unpacked + 13) // XXX maybe do not clear that much, just put 0 at the end while compressing
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!:
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sta tk__len_unpacked, y
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dey
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bpl !-
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// FALLTROUGH
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tk_pack_loop:
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// Get the next character
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lda BUF, x
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// If character cannot be a part of a keyword - do not pack anything more
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cmp #$23 // '#' - first sane character for a keyword
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bcc tk_pack_done // branch if below '#'
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cmp #$7B // 'Z' + 1
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bcs tk_pack_done // branch if above 'Z'
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// Keywords should not contain digits, colons, or semicolons
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cmp #$30
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bcc !+ // branch if below '0'
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cmp #$3C
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bcc tk_pack_done // branch if below '<'
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!:
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// Try to find a nibble to encode the character
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ldy #$0E
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!:
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cmp packed_as_1n-1, y
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beq tk_pack_1n
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dey
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bne !-
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// Try to find a 3-nibble sequence to encode the character (1st nibble is always 0xF in this case)
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ldy #TK__PACKED_AS_3N
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!:
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cmp packed_as_3n-1, y
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beq tk_pack_3n
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dey
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bne !-
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// If it is not on the list, it cannot be a part of token - quit
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// FALLTROUGH
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tk_pack_done:
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rts
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tk_pack_1n:
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// .Y contains our nibble; check whether we should start a new encoded byte
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bit tk__nibble_flag
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bpl tk_pack_1n_new_byte
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// Store in the high nibble of the current byte
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tya
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asl
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asl
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asl
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asl
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ldy tk__byte_offset
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ora tk__packed, y
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sta tk__packed, y
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// Adjust remaining counters / flags
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inc tk__byte_offset
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inc tk__nibble_flag // $FF -> $00
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// FALLTROUGH
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tk_pack_1n_done:
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// Put the bit mark - to indicate 1-nibble encoding
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clc
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ror tk__shorten_bits
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// FALLTROUGH
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tk_pack_loop_next:
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// Prepare for the next loop iteration - increment counters
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inx
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inc tk__len_unpacked
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// Check length, if it is OK to pack one more byte; quit if not
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lda tk__len_unpacked
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cmp #TK__MAX_KEYWORD_LEN
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bne tk_pack_loop
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rts
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tk_pack_1n_new_byte:
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// Store in the low nibble of the next byte
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tya
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ldy tk__byte_offset
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sta tk__packed, y
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// Adjust remaining counters / flags
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dec tk__nibble_flag // $00 -> $FF
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bmi tk_pack_1n_done
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tk_pack_3n:
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// .Y contains 2 nibbles to encode; check whether we should start a new encoded byte
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bit tk__nibble_flag
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bpl tk_pack_3n_new_byte
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// Store 1st nibble (alwys 0xF) in the high nibble of the current byte
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phy_trash_a
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lda #$F0
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ldy tk__byte_offset
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ora tk__packed, y
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sta tk__packed, y
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// Store byte in the next byte
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iny
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pla
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sta tk__packed, y
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// Adjust remaining counters / flags
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iny
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sty tk__byte_offset
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inc tk__nibble_flag // $FF -> $00
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// FALLTROUGH
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tk_pack_3n_done:
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// Put the bit mark - to indicate 3-nibble encoding
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sec
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ror tk__shorten_bits
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bcc tk_pack_loop_next // branch always
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tk_pack_3n_new_byte:
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// First store everything in the current byte, we will set '0xF' mark later
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tya
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ldy tk__byte_offset
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sta tk__packed, y
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// Now store just the low nibble in the next byte
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iny
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and #$0F
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sta tk__packed, y
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// Go back for a moment and set the '0xF' mark in the low nibble
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dey
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lda tk__packed, y
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ora #$0F
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sta tk__packed, y
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iny
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// Adjust remaining counters / flags
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iny
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sty tk__byte_offset
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dec tk__nibble_flag // $00 -> $FF
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bmi tk_pack_3n_done // branch always
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