added slice function

stdlib/list.go

@@ -19,6 +19,7 @@ package stdlib
 
 import (
     "fmt"
+    "strconv"
     "gitlab.com/whom/shs/ast"
     "gitlab.com/whom/shs/log"
 )
@@ -144,3 +145,103 @@ func Tail(input *ast.Token, vars ast.VarTable, funcs ast.FuncTable) *ast.Token {
 
     return iter.Copy()
 }
+
+/* Slice
+ * Takes 3 args and returns a list
+ * Arg 1: starting index of sublist
+ * Arg 2: end index of sublist
+ * Arg 3: source list
+ * returns sublist, or nil if non list applied, or nil if start or end arent INTEGERS
+ * first index in a list is 0
+ *
+ * Example: (slice 1 2 (1 2 3)) -> (2 3)
+ * Example: (slice 0 0 (1 2 3)) -> (1)
+ */
+func Slice(input *ast.Token, vars ast.VarTable, funcs ast.FuncTable) *ast.Token {
+    input = input.Eval(funcs, vars, false)
+    start := input
+    end := input.Next
+    source := end.Next
+
+    if start.Tag != ast.NUMBER || end.Tag != ast.NUMBER {
+        log.Log(log.ERR,
+            "start and end must both be integers",
+            "slice")
+        return nil
+    }
+
+    if source.Tag != ast.LIST {
+        log.Log(log.ERR,
+            "non-list as parameter to head",
+            "head")
+        return nil
+    }
+
+    st, err := strconv.ParseInt(start.Value(), 10, 64)
+    en, errr := strconv.ParseInt(end.Value(), 10, 64)
+
+    if err != nil {
+        log.Log(log.ERR,
+            "couldnt parse integer from start value: " + err.Error(),
+            "slice")
+        return nil
+    }
+
+    if errr != nil {
+        log.Log(log.ERR,
+            "couldnt parse integer from end value: " + errr.Error(),
+            "slice")
+        return nil
+    }
+
+    if st < 0 || en < 0 {
+        log.Log(log.ERR,
+            "both indices must be positive",
+            "slice")
+        return nil
+    }
+
+    if st > en {
+        log.Log(log.ERR,
+            "end index must be greater than start index",
+            "slice")
+        return nil
+    }
+
+    en = en - st
+    var inner *ast.Token
+    buildIter := &inner
+    sourceIter := source.Expand()
+
+    for st > 0 {
+        if sourceIter == nil {
+            log.Log(log.ERR,
+                "start index out of bounds",
+                "slice")
+            return nil
+        }
+
+        sourceIter = sourceIter.Next
+        st -= 1
+    }
+
+    for en >= 0 {
+        if sourceIter == nil {
+            log.Log(log.ERR,
+                "end index out of bounds",
+                "slice")
+            return nil
+        }
+
+        *buildIter = sourceIter.Copy()
+        (*buildIter).Next = nil
+
+        buildIter = &((*buildIter).Next)
+        sourceIter = sourceIter.Next
+        en -= 1
+    }
+
+    ret := &ast.Token{Tag: ast.LIST}
+    ret.Direct(inner)
+    return ret
+}