add StringList internal datatype

src/prolog/string_list.rs

@@ -0,0 +1,82 @@
+use prolog::tabled_rc::*;
+
+use std::cell::{Ref, RefCell};
+use std::cmp::Ordering;
+use std::hash::{Hash, Hasher};
+use std::ops::{Index, RangeTo};
+
+#[derive(PartialOrd, PartialEq, Ord, Eq)]
+pub struct StringListWrapper(RefCell<String>);
+
+impl Hash for StringListWrapper {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.borrow().hash(state)
+    }
+}
+
+// cursor is ignored if the double_quotes flag is set to atom
+#[derive(Clone)]
+pub struct StringList {
+    body: TabledRc<StringListWrapper>,
+    cursor: usize, // use this to generate a chars() iterator on the fly,
+                   // and skip over the first cursor chars. 
+    expandable: bool
+}
+
+impl Hash for StringList {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.borrow().hash(state)
+    }
+}
+
+impl PartialOrd for StringList {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering>
+    {
+        Some(self.body.cmp(&other.body))
+    }
+}
+
+impl Ord for StringList {
+    fn cmp(&self, other: &Self) -> Ordering
+    {
+        self.body.cmp(&other.body)
+    }
+}
+
+impl PartialEq for StringList {
+    fn eq(&self, other: &Self) -> bool
+    {
+        self.body == other.body
+    }
+}
+
+impl Eq for StringList {}
+
+impl StringList {
+    #[inline]
+    pub fn new(s: String, expandable: bool, string_tbl: TabledData<StringListWrapper>) -> Self {
+        let body = TabledRc::new(StringListWrapper(RefCell::new(s)), string_tbl);
+
+        StringList {
+            cursor: 0,
+            body,
+            expandable
+        }
+    }
+
+    #[inline]
+    pub fn tail(&self) -> Self {
+        let mut new_string_list = self.clone();
+        
+        if let Some(c) = self.borrow()[self.cursor ..].chars().next() {        
+            new_string_list.cursor += c.len_utf8();
+        }
+        
+        new_string_list
+    }
+
+    #[inline]
+    pub fn borrow(&self) -> Ref<String> {
+        self.body.0.borrow()
+    }
+}