X-Git-Url: http://git.subgeniuskitty.com/vvhitespace/.git/blobdiff_plain/66af3009b12b115d5744460c72f52e37fb6dee09..6fc33bfde9a18073ffbfdc23d3c8006a22054bca:/rationale.md

diff --git a/rationale.md b/rationale.md
index e2c5156..c847fab 100644
--- a/rationale.md
+++ b/rationale.md
@@ -1,34 +1,103 @@
-VVhitespace is descended from Whitespace, adding a vertical tab to the language
-along with some restrictions to ease implementation. The name is intended to
-embrace the spirit of Whitespace's visual incomprehensibility.
+While considering methods for further obfuscating Whitespace code, I realized
+that the Whitespace language reference didn't enforce any particular alignment
+or grouping for code execution. In theory this means one could create a
+sequence of Whitespace trits which represents different sequences of commands
+depending on exactly where execution begins.
+
+One implementation of this idea is the creation of a 'hidden' label. For
+example, the command sequence
+
+    PUSH 0; DROP; PUSH 46
+
+assembles as
+
+    SSSSN SNN SSSTSTTTSN
+
+which can be visually regrouped as
+
+    SSSSNSN NSSSTSTTTSN
+
+and contains the `MARK label0` command (i.e. `NSSSTSTTTSN`) used in the next
+set of examples.
+
+Additionally, since `PUSH 0; DROP` is effectively a `NOP`, 'hijacking' the code
+at this location allows one to insert their own integer on the stack in place
+of the `PUSH 46` command, sneakily substituting it as an input to any
+downstream processing.
+
+I decided to investigate the behavior of specific Whitespace interpreters,
+discovering that they broke down into two methods for locating labels.
+
+  * **Method 1** Scan from the start of the file for the first occurence of the
+    mark-label bytestring and jump.
+    
+    Examples:
+    
+        whitespacers/c: (c) meth0dz
+
+  * **Method 2** Scan from the start of the file, looking for a mark-label
+    bytestring, but 'parsing' one bytestring at a time, and jumping to the
+    first 'standalone' mark-label bytestring. Note that this is different than
+    executing the program, particularly when user-input commands are present.
+    
+    Examples:
+    
+        whitespacers/haskell: (c) 2003 Edwin Brady
+        whitespacers/ruby: (c) 2003 by Wayne E. Conrad
+        whitespacers/perl: (c) 2003 Micheal Koelbl
+        threeifbywhiskey/satan
+
+Both of these methods can be broken using valid Whitespace code:
+
+  * **Type A**: No 'standalone' label exists. This breaks Method 2.
+
+    By programmer's intent, this should print a `!` before infinite `.` lines.
+
+  * **Type B**: Hidden label before 'standalone' label. This breaks Method 1.
+
+    By programmer's intent, this should print an infinite chain of `.` lines.
+
+This is the Type A program:
+
+    SSSTSSSSTN  | Push +33 (ASCII '!')
+    NSNSTSTTTSN | JMP > 0101110 (label0)
+    NSSTTTTN    | MARK: 1111 (label2)
+    SSSSN       | PUSH 0
+    SNN         | DROP
+    SSSTSTTTSN  | Push +46 (ASCII '.')
+    TNSS        | Output character
+    SSSTSTSN    | Push +10 (ASCII '\n')
+    TNSS        | Output character
+    NSNTTTTN    | JMP > 1111 (label2)
+
+Append this to turn it into the Type B program:
+
+    NSSSTSTTTSN | MARK: 0101110 (label0) (2nd time)
+    NSNTTTTN    | JMP > 1111 (label2)
+
+VVhitespace avoids this ambiguity by marking label definitions with a vertical
+tab `[VTab]` immediately before the label.
+
+    Old label: NSS  TSTS N
+    New label: NSSV TSTS N
 
 Since Whitespace ignores [VTab] as a comment character, and since the
 Whitespace VM is a superset of the VVhitespace VM, all valid VVhitespace
-programs are also valid Whitespace programs.
-
-Locating a Whitespace interpreter capable of running all valid Whitespace
-programs is left as an exercise for the reader. For the inquisitive, the
-following program, read top-down, is both a valid VVhitespace and Whitespace
-program. Does it terminate or infinitely loop?
-
-[Space]
-[Space]
-[Space]
-[Tab]
-[LF]
-[LF]
-[Space]
-[LF]
-[Space]
-[Tab]
-[LF]
-[LF]
-[Space]
-[Space]
-[VTab]
-[Space]
-[Tab]
-[LF]
-[LF]
-[LF]
-[LF]
+programs are also valid Whitespace programs, though the task of locating a
+suitable Whitespace interpreter is left for the reader.
+
+--------------------------------------------------------------------------------
+
+Quoting from the original Whitespace tutorial which I used as language reference:
+
+    The programmer is free to push arbitrary width integers onto the stack.
+
+I have yet to find a Whitespace interpreter which successfully implements that
+statement.
+
+Since I wanted to implement bitwise logic functions, this broad definition
+posed a problem. For example, how many `1`s should be in the output of the
+expression `NOT(0)`? Should the expression `NOT(0) == NOT(0)` always be true?
+
+VVhitespace sidesteps the problem by declaring all integers to be 64-bits wide.
+