From a1e33af5a04595ed3d6d090f21bc3d40e035e4e2 Mon Sep 17 00:00:00 2001
From: Stephen Chung <Stephen.Chung@intexact.com>
Date: Mon, 20 Apr 2020 11:08:54 +0800
Subject: [PATCH] Reduce size of Position by limiting resolution to 16 bits.

---
 src/token.rs | 37 ++++++++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 13 deletions(-)

diff --git a/src/token.rs b/src/token.rs
index aa11a3c6..e65e5e3e 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -13,29 +13,29 @@ use crate::stdlib::{
     iter::Peekable,
     str::{Chars, FromStr},
     string::{String, ToString},
-    usize,
+    u16,
     vec::Vec,
 };
 
 type LERR = LexError;
 
 /// A location (line number + character position) in the input script.
+///
+/// In order to keep footprint small, both line number and character position have 16-bit resolution,
+/// meaning they go up to a maximum of 65,535 lines/characters per line.
+/// Advancing beyond the maximum line length or maximum number of lines is not an error but has no effect.
 #[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Clone, Copy)]
 pub struct Position {
     /// Line number - 0 = none
-    line: usize,
+    line: u16,
     /// Character position - 0 = BOL
-    pos: usize,
+    pos: u16,
 }
 
 impl Position {
     /// Create a new `Position`.
-    pub fn new(line: usize, position: usize) -> Self {
+    pub fn new(line: u16, position: u16) -> Self {
         assert!(line != 0, "line cannot be zero");
-        assert!(
-            line != usize::MAX || position != usize::MAX,
-            "invalid position"
-        );
 
         Self {
             line,
@@ -48,7 +48,7 @@ impl Position {
         if self.is_none() {
             None
         } else {
-            Some(self.line)
+            Some(self.line as usize)
         }
     }
 
@@ -57,13 +57,18 @@ impl Position {
         if self.is_none() || self.pos == 0 {
             None
         } else {
-            Some(self.pos)
+            Some(self.pos as usize)
         }
     }
 
     /// Advance by one character position.
     pub(crate) fn advance(&mut self) {
-        self.pos += 1;
+        assert!(!self.is_none(), "cannot advance Position::none");
+
+        // Advance up to maximum position
+        if self.pos < u16::MAX {
+            self.pos += 1;
+        }
     }
 
     /// Go backwards by one character position.
@@ -73,14 +78,20 @@ impl Position {
     /// Panics if already at beginning of a line - cannot rewind to a previous line.
     ///
     pub(crate) fn rewind(&mut self) {
+        assert!(!self.is_none(), "cannot rewind Position::none");
         assert!(self.pos > 0, "cannot rewind at position 0");
         self.pos -= 1;
     }
 
     /// Advance to the next line.
     pub(crate) fn new_line(&mut self) {
-        self.line += 1;
-        self.pos = 0;
+        assert!(!self.is_none(), "cannot advance Position::none");
+
+        // Advance up to maximum position
+        if self.line < u16::MAX {
+            self.line += 1;
+            self.pos = 0;
+        }
     }
 
     /// Create a `Position` representing no position.