diff --git a/src/cmap/subtable12.rs b/src/cmap/subtable12.rs
index 88aad4a4..a566fc11 100644
--- a/src/cmap/subtable12.rs
+++ b/src/cmap/subtable12.rs
@@ -105,7 +105,7 @@ pub(crate) fn subset_subtable12(ctx: &Context, data: &[u8]) -> crate::Result<Vec
         .filter_map(|c| {
             if let Some(g) = subtable.glyph_index(c) {
                 if ctx.requested_glyphs.contains(&g) {
-                    if let Some(new_g) = ctx.gid_map.get(&g) {
+                    if let Some(new_g) = ctx.mapper.forward.get(&g) {
                         return Some((c, *new_g));
                     }
                 }
diff --git a/src/cmap/subtable4.rs b/src/cmap/subtable4.rs
index f67ca314..275dd3b7 100644
--- a/src/cmap/subtable4.rs
+++ b/src/cmap/subtable4.rs
@@ -159,7 +159,7 @@ pub(crate) fn subset_subtable4(ctx: &Context, data: &[u8]) -> crate::Result<Vec<
         .filter_map(|c| {
             if let Some(g) = subtable.glyph_index(c as u32) {
                 if ctx.requested_glyphs.contains(&g) {
-                    if let Some(new_g) = ctx.gid_map.get(&g) {
+                    if let Some(new_g) = ctx.mapper.forward.get(&g) {
                         return Some((c, *new_g));
                     }
                 }
diff --git a/src/glyf.rs b/src/glyf.rs
index 394b073e..1f8046c0 100644
--- a/src/glyf.rs
+++ b/src/glyf.rs
@@ -132,7 +132,7 @@ pub(crate) fn subset(ctx: &mut Context) -> Result<()> {
         }
     };
 
-    for old_gid in &ctx.reverse_gid_map {
+    for old_gid in &ctx.mapper.backward {
         write_offset(sub_glyf.len());
         let data = table.glyph_data(*old_gid)?;
 
@@ -185,7 +185,7 @@ fn remap_component_glyphs(ctx: &Context, data: &[u8]) -> Result<Vec<u8>> {
         let flags = r.read::<u16>()?;
         w.write(flags);
         let component = r.read::<u16>()?;
-        let new_component = *ctx.gid_map.get(&component).ok_or(Error::InvalidData)?;
+        let new_component = *ctx.mapper.forward.get(&component).ok_or(Error::InvalidData)?;
         w.write(new_component);
 
         if flags & ARG_1_AND_2_ARE_WORDS != 0 {
diff --git a/src/hmtx.rs b/src/hmtx.rs
index bd659c29..958130aa 100644
--- a/src/hmtx.rs
+++ b/src/hmtx.rs
@@ -17,7 +17,7 @@ pub(crate) fn subset(ctx: &mut Context) -> Result<()> {
 
     let mut last_advance_width = 0;
     for i in 0..ctx.subset.len() {
-        let original_gid = ctx.reverse_gid_map[i];
+        let original_gid = ctx.mapper.backward[i];
 
         if original_gid < num_h_metrics {
             let offset = original_gid as usize * 4;
diff --git a/src/lib.rs b/src/lib.rs
index 72078255..ab517eb0 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -48,11 +48,15 @@ mod maxp;
 mod name;
 mod post;
 mod stream;
+mod mapper;
 
 use crate::stream::{Reader, Structure, Writer};
 use std::borrow::Cow;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::fmt::{self, Debug, Display, Formatter};
+use crate::mapper::{InternalMapper, MapperVariant};
+
+pub use crate::mapper::Mapper;
 
 /// Subset a font face to include less glyphs and tables.
 ///
@@ -63,7 +67,7 @@ pub fn subset(
     data: &[u8],
     index: u32,
     profile: &[u16],
-) -> Result<(Vec<u8>, HashMap<u16, u16>)> {
+) -> Result<(Vec<u8>, Mapper)> {
     let face = parse(data, index)?;
     let kind = match face.table(Tag::CFF).or(face.table(Tag::CFF2)) {
         Some(_) => FontKind::Cff,
@@ -82,8 +86,7 @@ pub fn subset(
         num_glyphs,
         subset: HashSet::new(),
         requested_glyphs,
-        gid_map: HashMap::new(),
-        reverse_gid_map: vec![],
+        mapper: InternalMapper::new(),
         kind,
         tables: vec![],
         long_loca: true,
@@ -164,7 +167,7 @@ fn parse(data: &[u8], index: u32) -> Result<Face<'_>> {
 }
 
 /// Construct a brand new font.
-fn construct(mut ctx: Context) -> (Vec<u8>, HashMap<u16, u16>) {
+fn construct(mut ctx: Context) -> (Vec<u8>, Mapper) {
     let mut w = Writer::new();
     w.write::<FontKind>(ctx.kind);
 
@@ -228,7 +231,7 @@ fn construct(mut ctx: Context) -> (Vec<u8>, HashMap<u16, u16>) {
         data[i..i + 4].copy_from_slice(&val.to_be_bytes());
     }
 
-    (data, ctx.gid_map)
+    (data, Mapper(MapperVariant::HashmapMapper(ctx.mapper)))
 }
 
 /// Calculate a checksum over the sliced data as a sum of u32s. If the data
@@ -255,11 +258,8 @@ struct Context<'a> {
     /// Actual glyphs that are needed to subset the font correctly,
     /// including glyphs referenced indirectly through components.
     subset: HashSet<u16>,
-    // A map from old gids to new gids
-    gid_map: HashMap<u16, u16>,
-    // A map from new gids to old gids. The index represents the
-    // new gid, and the value at that index the old gid.
-    reverse_gid_map: Vec<u16>,
+    // A map from old gids to new gids, and the reverse
+    mapper: InternalMapper,
     /// The kind of face.
     kind: FontKind,
     /// Subsetted tables.
@@ -303,8 +303,8 @@ impl<'a> Context<'a> {
         original_gids.sort();
 
         for (counter, gid) in original_gids.into_iter().enumerate() {
-            self.gid_map.insert(*gid, counter as u16);
-            self.reverse_gid_map.push(*gid);
+            self.mapper.forward.insert(*gid, counter as u16);
+            self.mapper.backward.push(*gid);
         }
     }
 
diff --git a/src/mapper.rs b/src/mapper.rs
new file mode 100644
index 00000000..56501613
--- /dev/null
+++ b/src/mapper.rs
@@ -0,0 +1,41 @@
+use std::collections::HashMap;
+
+#[derive(Debug, Clone)]
+pub(crate) struct InternalMapper {
+    pub(crate) forward: HashMap<u16, u16>,
+    pub(crate) backward: Vec<u16>
+}
+
+impl InternalMapper {
+    pub fn new() -> Self {
+        Self {
+            forward: HashMap::new(),
+            backward: Vec::new()
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub(crate) enum MapperVariant {
+    IdentityMapper,
+    HashmapMapper(InternalMapper)
+}
+
+/// A mapper that maps old gids to new ones.
+#[derive(Debug, Clone)]
+pub struct Mapper(pub(crate) MapperVariant);
+
+impl Mapper {
+    /// Create a mapper that maps each gid to itself.
+    pub fn identity_mapper() -> Self {
+        Self(MapperVariant::IdentityMapper)
+    }
+
+    /// Get the newly mapped gid for an old gid.
+    pub fn get(&self, gid: u16) -> Option<u16> {
+        match self.0 {
+            MapperVariant::IdentityMapper => Some(gid),
+            MapperVariant::HashmapMapper(ref h) => h.forward.get(&gid).copied()
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/post.rs b/src/post.rs
index 25fd0d0e..186d5529 100644
--- a/src/post.rs
+++ b/src/post.rs
@@ -40,7 +40,7 @@ pub(crate) fn subset(ctx: &mut Context) -> Result<()> {
     let mut sub_strings = Writer::new();
     let mut count = 0;
     for i in 0..num_glyphs {
-        let old_gid = ctx.reverse_gid_map[i as usize];
+        let old_gid = ctx.mapper.backward[i as usize];
         let index = indices[old_gid as usize];
 
         if index <= 257 {
diff --git a/tests/src/main.rs b/tests/src/main.rs
index 6d0cc17e..04ea092c 100644
--- a/tests/src/main.rs
+++ b/tests/src/main.rs
@@ -1,9 +1,8 @@
 use sha2::{Digest, Sha256};
-use std::collections::HashMap;
 use std::error::Error;
 use std::fs;
 use std::path::PathBuf;
-use subsetter::subset;
+use subsetter::{Mapper, subset};
 use ttf_parser::GlyphId;
 
 #[rustfmt::skip]
@@ -11,12 +10,12 @@ mod ttf;
 
 type Result<T> = std::result::Result<T, Box<dyn Error>>;
 
-const SAVE_SUBSETS: bool = true;
+const SAVE_SUBSETS: bool = false;
 
 struct TestContext {
     font: Vec<u8>,
     subset: Vec<u8>,
-    gid_map: HashMap<u16, u16>,
+    mapper: Mapper,
     gids: Vec<u16>,
 }
 
@@ -42,13 +41,13 @@ fn get_test_context(font_file: &str, gids: &str) -> Result<TestContext> {
     let data = std::fs::read(font_path)?;
     let gids: Vec<_> = parse_gids(gids);
 
-    let (subset, gid_map) = subset(&data, 0, &gids)?;
+    let (subset, mapper) = subset(&data, 0, &gids)?;
 
     if SAVE_SUBSETS {
         save_font(&subset);
     }
 
-    Ok(TestContext { font: data, subset, gid_map, gids })
+    Ok(TestContext { font: data, subset, mapper: mapper, gids })
 }
 
 fn parse_gids(gids: &str) -> Vec<u16> {
@@ -96,7 +95,7 @@ fn cmap(font_file: &str, gids: &str) {
         .collect::<Vec<_>>();
 
     for (c, gid) in relevant_chars {
-        let mapped_gid = ctx.gid_map.get(&gid.0).copied();
+        let mapped_gid = ctx.mapper.get(gid.0);
         let cur_gid = new_face.glyph_index(c).map(|g| g.0);
         assert_eq!((c, mapped_gid), (c, cur_gid));
     }
@@ -139,7 +138,7 @@ fn glyph_metrics(font_file: &str, gids: &str) {
         .copied()
         .filter(|g| ctx.gids.contains(g) && *g < old_face.number_of_glyphs())
     {
-        let mapped = *ctx.gid_map.get(&glyph).unwrap();
+        let mapped = ctx.mapper.get(glyph).unwrap();
 
         assert_eq!(
             old_face.glyph_bounding_box(GlyphId(glyph)),
@@ -177,7 +176,7 @@ pub fn glyph_outlines(font_file: &str, gids: &str) {
         let mut sink1 = Sink::default();
         let mut sink2 = Sink::default();
         old_face.outline_glyph(GlyphId(glyph), &mut sink1);
-        new_face.outline_glyph(GlyphId(*ctx.gid_map.get(&glyph).unwrap()), &mut sink2);
+        new_face.outline_glyph(GlyphId(ctx.mapper.get(glyph).unwrap()), &mut sink2);
         assert_eq!(sink1, sink2);
     }
 }