diff --git a/source/durandal/bin.rs b/source/durandal/bin.rs
index 6f15769..6a12bc4 100644
--- a/source/durandal/bin.rs
+++ b/source/durandal/bin.rs
@@ -156,17 +156,26 @@ macro_rules! _durandal_read_impl {
 /// #    Ok(())
 /// # }
 /// ```
-#[macro_export(local_inner_macros)]
+#[macro_export]
 macro_rules! read_data {
    (
       $sz:expr , $ty:ident in $b:ident =>
       $( $nam:ident = $t:ident [ $n:expr ] $( $ex:ident )* ; )*
+   ) => {
+      $crate::check_data!($sz, $b);
+      $($crate::_durandal_read_impl!($ty $b $nam $($ex)* $t $n);)*
+   };
+}
+
+/// Checks if there is enough data in `b`.
+#[macro_export]
+macro_rules! check_data {
+   (
+      $sz:expr , $b:ident
    ) => {
       if $b.len() < $sz {
          return Err(err_msg("not enough data"));
       }
-
-      $($crate::_durandal_read_impl!($ty $b $nam $($ex)* $t $n);)*
    };
 }
 
@@ -360,11 +369,7 @@ pub fn rd_ofstable<T, F>(b: &[u8],
 
    for _ in 0..num {
       let ofs = usize_from_u32(u32b(&b[p..p + 4]));
-
-      if ofs >= b.len() {
-         bail!("not enough data");
-      }
-
+      check_data!(ofs, b);
       v.push(read(&b[ofs..])?);
       p += 4;
    }
diff --git a/source/durandal/bit.rs b/source/durandal/bit.rs
index 72dea3d..3ca1b5b 100644
--- a/source/durandal/bit.rs
+++ b/source/durandal/bit.rs
@@ -2,53 +2,49 @@
 
 use crate::durandal::err::*;
 
-/// Reads `width` bits from `b` starting at bit `cr_ptr` into an integer.
+/// Reads `width` bits from `b` starting at bit `cr_bit` into an integer.
 ///
 /// # Errors
 ///
 /// The function will return an error if `width` is 0 or more than 64, or if
-/// `(cr_ptr + width - 1) / 8` would overflow an index into `b`.
-pub fn read_bits(b: &[u8], cr_ptr: usize, width: u8) -> ResultS<(usize, u64)>
+/// `(cr_bit + width - 1) / 8` would overflow an index into `b`.
+pub fn read_bits(b: &[u8], cr_bit: usize, width: u8) -> ResultS<(usize, u64)>
 {
    if width < 1 || width > 64 {
       bail!("invalid number of bits");
    }
 
-   let nx_ptr = cr_ptr + usize::from(width);
-   let ed_ptr = nx_ptr - 1;
+   let nx_bit = cr_bit + usize::from(width);
+   let ed_bit = nx_bit - 1;
 
-   let first_byte = cr_ptr / 8;
-   let last_byte  = ed_ptr / 8;
+   let cr_byte = cr_bit / 8;
+   let ed_byte = ed_bit / 8;
 
-   if b.len() <= last_byte {
-      bail!("not enough data");
-   }
+   check_data!(ed_byte + 1, b);
 
-   let first_bits = cr_ptr as u32 % 8;
-   let last_bits  = nx_ptr as u32 % 8;
+   let cr_bits = cr_bit as u32 % 8;
+   let nx_bits = nx_bit as u32 % 8;
 
-   let last_mask = ((1_u128 << last_bits) - 1) as u64;
+   let nx_mask = ((1_u128 << nx_bits) - 1) as u64;
 
-   let num_bytes = last_byte - first_byte;
+   let num_bytes = ed_byte - cr_byte;
 
-   let bytes = get_bytes(b, num_bytes, first_byte);
+   let bytes = get_bytes(b, num_bytes, cr_byte);
 
    let bits = u64::from_be_bytes(bytes);
-   let bits = bits.wrapping_shl(first_bits);
+   let bits = bits.wrapping_shl(cr_bits);
    let bits = bits >> (64 - width);
 
    // special case for over byte boundary
-   let bits = if num_bytes == 8 {
-      let lbyt = u64::from(b[last_byte]);
-      let lbyt = lbyt >> (8 - last_bits);
-      let lbyt = lbyt & last_mask;
+   if num_bytes == 8 {
+      let lbyt = u64::from(b[ed_byte]);
+      let lbyt = lbyt >> (8 - nx_bits);
+      let lbyt = lbyt & nx_mask;
 
-      (bits & !last_mask) | lbyt
+      Ok((nx_bit, bits & !nx_mask | lbyt))
    } else {
-      bits
-   };
-
-   Ok((nx_ptr, bits))
+      Ok((nx_bit, bits))
+   }
 }
 
 fn get_bytes(b: &[u8], num_bytes: usize, f: usize) -> [u8; 8]
diff --git a/source/marathon/defl.rs b/source/marathon/defl.rs
index b07bd3b..ddc8d8c 100644
--- a/source/marathon/defl.rs
+++ b/source/marathon/defl.rs
@@ -25,10 +25,7 @@ pub fn load_gzip_header(b: &[u8]) -> ResultS<usize>
 
    if fl & FEXTRA != 0 {
       read_data!(p + 2, LE in b => xlen = u16[p] usize;);
-
-      if p + 2 + xlen > b.len() {
-         bail!("not enough data");
-      }
+      check_data!(p + 2 + xlen, b);
 
       p += xlen;
    }
@@ -42,7 +39,7 @@ pub fn load_gzip_header(b: &[u8]) -> ResultS<usize>
    }
 
    if fl & FHCRC != 0 {
-      read_data!(p + 2, LE in b => _crc = u16[p];);
+      check_data!(p + 2, b);
 
       p += 2;
    }
diff --git a/source/marathon/map.rs b/source/marathon/map.rs
index 7d4a9d8..81f4724 100644
--- a/source/marathon/map.rs
+++ b/source/marathon/map.rs
@@ -88,9 +88,7 @@ pub fn read_old_minf(b: &[u8]) -> ResultS<Minf>
 /// Reads an `iidx` chunk.
 pub fn read_iidx(b: &[u8]) -> ResultS<(u16, usize)>
 {
-   if b.len() < 2 {
-      bail!("not enough data");
-   }
+   check_data!(2, b);
 
    Ok((u16b(b), 2))
 }
diff --git a/source/marathon/shp.rs b/source/marathon/shp.rs
index 276ffc1..4df8363 100644
--- a/source/marathon/shp.rs
+++ b/source/marathon/shp.rs
@@ -88,11 +88,12 @@ pub fn read_bitmap(b: &[u8]) -> ResultS<Bitmap>
             fst = u16[p] usize;
             lst = u16[p + 2] usize;
          }
+
          let end = lst - fst;
 
          p += 4;
 
-         if lst < fst || fst > pitch || lst > pitch || b.len() < p + end {
+         if lst < fst || fst > pitch || lst > pitch {
             bail!("invalid compressed scanline");
          }
 
@@ -110,10 +111,6 @@ pub fn read_bitmap(b: &[u8]) -> ResultS<Bitmap>
       }
    } else {
       // simple copy
-      if b.len() < p + width * height {
-         bail!("invalid scanline");
-      }
-
       bmp.cr.extend_from_slice(ok!(b.get(p..p + width * height),
                                    "not enough data")?);
    }