[project @ 2004-08-13 13:04:50 by simonmar]

[ghc-hetmet.git] / ghc / compiler / utils / FastString.lhs

diff --git a/ghc/compiler/utils/FastString.lhs b/ghc/compiler/utils/FastString.lhs
index 494648f..8dbfefa 100644 (file)
--- a/ghc/compiler/utils/FastString.lhs
+++ b/ghc/compiler/utils/FastString.lhs
@@ -3,8 +3,18 @@
 %
 \section{Fast strings}
 
-Compact representations of character strings with
-unique identifiers (hash-cons'ish).
+FastString:    A compact, hash-consed, representation of character strings.
+               Comparison is O(1), and you can get a Unique from them.
+               Generated by the FSLIT macro
+               Turn into SDoc with Outputable.ftext
+
+LitString:     Just a wrapper for the Addr# of a C string (Ptr CChar).
+               Practically no operations
+               Outputing them is fast
+               Generated by the SLIT macro
+               Turn into SDoc with Outputable.ptext
+
+Use LitString unless you want the facilities of FastString
 
 \begin{code}
 module FastString
@@ -106,6 +116,7 @@ instance Eq FastString where
   a /= b = case cmpFS a b of { LT -> True;  EQ -> False; GT -> True  }
 
 instance Ord FastString where
+       -- Compares lexicographically, not by unique
     a <= b = case cmpFS a b of { LT -> True;  EQ -> True;  GT -> False }
     a <         b = case cmpFS a b of { LT -> True;  EQ -> False; GT -> False }
     a >= b = case cmpFS a b of { LT -> False; EQ -> True;  GT -> True  }
@@ -391,9 +402,9 @@ hashSubStrBA ba# start# len# =
    2# -> ((ord# c0 *# 631#) +# (ord# c1 *# 217#) +# len#) `remInt#` hASH_TBL_SIZE#
    _  -> ((ord# c0 *# 631#) +# (ord# c1 *# 217#) +# (ord# c2 *# 43#) +# len#) `remInt#` hASH_TBL_SIZE#
   where
-    c0 = indexCharArray# ba# 0#
-    c1 = indexCharArray# ba# (len# `quotInt#` 2# -# 1#)
-    c2 = indexCharArray# ba# (len# -# 1#)
+    c0 = indexCharArray# ba# (start# +# 0#)
+    c1 = indexCharArray# ba# (start# +# (len# `quotInt#` 2# -# 1#))
+    c2 = indexCharArray# ba# (start# +# (len# -# 1#))
 
 --    c1 = indexCharArray# ba# 1#
 --    c2 = indexCharArray# ba# 2#
@@ -497,7 +508,6 @@ hPutFS handle (UnicodeStr _ is)
 -- LitStrings, here for convenience only.
 
 type LitString = Ptr ()
--- ToDo: make it a Ptr when we don't have to support 4.08 any more
 
 mkLitString# :: Addr# -> LitString
 mkLitString# a# = Ptr a#