rand: Add an example with PRNG

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

1 files changed, 55 insertions, 0 deletions

diff --git a/rand/precalc.s b/rand/precalc.s
new file mode 100644
index 0000000..0b6da86
--- /dev/null
+++ b/rand/precalc.s
@@ -0,0 +1,55 @@
+;;;
+;; Pseudo-random number implementation by using a pre-calculated table.
+;;
+;; The implementation for this is extremely simple, as the given input parameter
+;; is used to index the random table below. Note that the table is 256 bytes
+;; long. This has the benefit that you never need to worry about bad indexing,
+;; and that the implementation is kept simple and fast. The inconvenience is
+;; that, obviously, you lose 256 bytes of data.
+;;
+;; From a programmer's happiness perspective, this solution also feels quite
+;; uninspired, but it gets the job done and there will be no surprises.
+;;
+;; From the player's perspective, of course, if they are messing with the code
+;; itself, it's pretty easy to manipulate the randomness of it all. As in, if
+;; they are using a TAS or they have mad frame-perfect input skills, they can
+;; predict the seed and get the value they want.
+;;
+;; All of the above being said, Final Fantasy is famous for using this technique
+;; for their random number generation, so it's not as unhinged as you would
+;; think.
+;;
+;; NOTE: to avoid the cost of having to have a 256 byte table, one idea I've had
+;; is that instead of using the `rand_table` we could use simply `main` (or
+;; better yet, `$8000`), as that spans more than 256 bytes. Moreover, the linker
+;; can have a configuration for random fill values, so any gaps could also have
+;; random bytes in it.
+
+;; Given a number passed via the 'a' register, it updates this same 'a' register
+;; with the next random number.
+;;
+;; NOTE: implemented by indexing a pre-calculated random table.
+.proc precalc
+    tax
+    lda rand_table, x
+    rts
+.endproc
+
+;; Pre-computed table, hopefully it feels random enough :)
+rand_table:
+    .byte $D7, $3A, $1C, $8F, $09, $B2, $E6, $54, $A3, $91, $2B, $F5, $78, $0D, $4C, $6E
+    .byte $FF, $C0, $52, $33, $6A, $E9, $9B, $1A, $47, $88, $7D, $21, $0E, $F4, $B3, $9C
+    .byte $15, $67, $A8, $41, $D2, $39, $80, $76, $C9, $E5, $0A, $1B, $5F, $22, $73, $DA
+    .byte $B4, $96, $3C, $E0, $8D, $F7, $2A, $05, $9E, $43, $11, $6D, $A7, $58, $C1, $32
+    .byte $28, $0F, $79, $BE, $51, $64, $9D, $A9, $3B, $71, $8E, $C6, $4A, $13, $F0, $27
+    .byte $E2, $5C, $06, $D3, $95, $B8, $4F, $70, $19, $A4, $6B, $38, $82, $C7, $5E, $01
+    .byte $F3, $2D, $9A, $65, $7C, $D1, $0B, $E8, $57, $36, $84, $1F, $B0, $92, $45, $AC
+    .byte $60, $7E, $A1, $53, $C8, $29, $D4, $FB, $07, $42, $E3, $99, $16, $8A, $3D, $C5
+    .byte $24, $B1, $6F, $03, $7A, $E7, $8C, $59, $D0, $46, $93, $1E, $A5, $2C, $B7, $F1
+    .byte $89, $55, $C3, $30, $62, $98, $04, $D6, $7F, $A0, $E4, $12, $3B, $81, $F9, $23
+    .byte $C4, $0D, $5A, $71, $9F, $B6, $2E, $85, $37, $A9, $18, $6C, $E1, $4B, $D9, $02
+    .byte $F8, $63, $B5, $40, $97, $0C, $7A, $51, $A2, $3E, $8F, $D5, $14, $69, $E0, $B8
+    .byte $4D, $77, $25, $9B, $0A, $F2, $3C, $86, $E9, $1F, $68, $A3, $50, $C1, $7D, $04
+    .byte $B2, $8E, $56, $1D, $73, $9C, $F5, $2A, $61, $D7, $09, $3E, $84, $A0, $E6, $1B
+    .byte $3F, $C8, $94, $05, $72, $D6, $A7, $4C, $1A, $5F, $B3, $29, $80, $E1, $6D, $9E
+    .byte $0C, $43, $F7, $8B, $52, $16, $A8, $3D, $91, $2B, $E5, $70, $C6, $4A, $D9, $F8