negative number parsing

ast/lex.go

@@ -215,6 +215,11 @@ error:
 func StrIsNumber(arg string) bool {
     dotCount := 0
 
+    // negative nums
+    if len(arg) > 0 && arg[0] == '-' {
+        arg = arg[1:]
+    }
+
     for _, char := range arg {
         if !unicode.IsDigit(char) {
             if char == '.' && dotCount == 0 {

stdlib/stdlib.go

@@ -157,6 +157,13 @@ func GenFuncTable() ast.FuncTable {
             Args:           2,
         },
 
+        "substr": &ast.Function{
+            Function:  Substr,
+            Name:    "substr",
+            TimesCalled:    0,
+            Args:           3,
+        },
+
         "exit": &ast.Function{
             Function:  ExitShell,
             Name:         "exit",

stdlib/string.go

@@ -145,6 +145,80 @@ func Join(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
     return ret
 }
 
+/* takes three arguments:
+ *   1. start index
+ *   2. end index
+ *   3. source
+ * Returns a substring from source delimited by args 1 and 2.
+ * First two args must be integers (4 or 4.0 but not 4.3)
+ *
+ * Example: (substr 1 5 "Linus Torvalds") -> "inus "
+ */
+func Substr(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
+    start := in.Eval(ft, vt, false)
+    if start == nil || start.Next == nil || start.Next.Next == nil {
+        log.Log(log.ERR,
+            "an argument evaluated to nil",
+            "substr")
+        return nil
+    }
+
+    end := start.Next
+    str := end.Next
+
+    if start.Tag != ast.NUMBER || end.Tag != ast.NUMBER || str.Tag != ast.STRING {
+        log.Log(log.ERR,
+            "incorrect types of args",
+            "substr")
+        return nil
+    }
+
+    ed_idx := 0
+    st_idx, err := strconv.Atoi(start.Value())
+    ed_idx, err = strconv.Atoi(end.Value())
+    if err != nil {
+        log.Log(log.ERR,
+            "error parsing args: " + err.Error(),
+            "substr")
+        return nil
+    }
+
+    strlen := len(str.Value())
+    if st_idx < 0 {
+        st_idx += strlen
+    }
+
+    if ed_idx < 0 {
+        ed_idx += strlen
+    }
+
+    if st_idx < 0 || st_idx >= strlen {
+        log.Log(log.ERR,
+            "first index out of bounds",
+            "substr")
+        return nil
+    }
+
+    if ed_idx < 0 || ed_idx >= strlen {
+        log.Log(log.ERR,
+            "last index out of bounds",
+            "substr")
+        return nil
+    }
+
+    if st_idx > ed_idx {
+        log.Log(log.ERR,
+            "start must be less than end",
+            "substr")
+        return nil
+    }
+
+    res := str.Value()[st_idx:ed_idx]
+    ret := &ast.Token{Tag: ast.STRING}
+    ret.Set(res)
+    return ret
+}
+
 /* Takes one arg, returns nil
  * Prints a string to stdout
  * Unquotes string so user can add escaped chars like \n, \t, etc