Implement strat.

PRIMITIVES.md

@@ -175,6 +175,9 @@ added to them too.
     * Returns file information as a list (TYPE SIZE MODE), or nil on failure.
   * `stdlib`
      * Return the contents of our slisp stdlib at runtime.
+  * `strat`
+    * Read the byte at offset N from the given string.
+    * This is a dangerous function, with no bounds-checking, which works around the fact `explode` and `substr` get foiled by embedded NULL characters.
   * `strcat`
     * Join two strings together and return them.
   * `strcmp`

compiler/template.tmpl

@@ -2770,6 +2770,40 @@ FUNC fn_sys_stdlib
     TAG_STRING_REG rax
     ret
 
+;; Return the given byte from the string, as an integer.
+;;
+;; This function is dangerous as it has no bound checking, we could add
+;; it since we know the size of all allocations made with alloc..
+;;
+;; For the moment it is a workaround for the fact our strings can't have
+;; embedded null-characters - if they do "explode" and "substr" don't work
+;; any longer.
+;;
+;; TODO: Add bound-checking.
+;;
+FUNC fn_sys_strat
+    mov rbx, rdi             ; type-check arg1
+    GET_TAG_BITS rbx
+    cmp rbx, TAG_ID_STRING
+    jne type_error
+    mov rbx, rsi             ; type-check arg2
+    GET_TAG_BITS rbx
+    cmp rbx, TAG_ID_INTEGER
+    jne type_error
+
+    UNTAG_REG rdi
+    UNTAG_REG rsi
+
+    ; RDI points to the string, add the RSI/offset to it
+    add rdi, rsi
+
+    ; Load and return the character/byte as an integer.
+    xor rax, rax
+    mov al, byte [rdi]
+    TAG_INTEGER_REG rax
+    ret
+
+
 ;; join the two given strings and return a new one.
 FUNC fn_sys_strcat
     mov rbx, rdi

inception.lisp

@@ -336,6 +336,7 @@
   (register-builtin "sys_sqrt" (lambda (args) (sys_sqrt (car args))))
   (register-builtin "sys_stat" (lambda (args) (sys_stat (car args))))
   (register-builtin "sys_stdlib" (lambda (args) (sys_stdlib)))
+  (register-builtin "sys_strat" (lambda (args) (sys_strat (car args) (cadr args))))
   (register-builtin "sys_strcat" (lambda (args) (sys_strcat (car args) (cadr args))))
   (register-builtin "sys_strcmp" (lambda (args) (sys_strcmp (car args) (cadr args))))
   (register-builtin "sys_strdup" (lambda (args) (sys_strdup (car args))))

stdlib.slisp

@@ -213,6 +213,12 @@ This returns a list of the form (TYPE SIZE MODE)."
   "Return the contents of the stdlib.slisp standard-library."
   (sys_stdlib))
 
+(defun strat(str n)
+  "Return the integer value of the character at offset N for the given string.
+
+This ignores embedded NULLs which would otherwise break substr and explode."
+  (sys_strat str n))
+
 (defun strcat(a b)
   "Return the result of concatenating the two specified strings into one."
   (sys_strcat a b))