src/HOL/Real/float_arith.ML
author nipkow
Tue Oct 23 23:27:23 2007 +0200 (2007-10-23)
changeset 25162 ad4d5365d9d8
parent 24630 351a308ab58d
child 25300 bc3a1c964704
permissions -rw-r--r--
went back to >0
     1 (*  Title:  HOL/Real/float_arith.ML
     2     ID:     $Id$
     3     Author: Steven Obua
     4 *)
     5 
     6 signature FLOAT_ARITH =
     7 sig
     8   exception Destruct_floatstr of string
     9   val destruct_floatstr: (char -> bool) -> (char -> bool) -> string -> bool * string * string * bool * string
    10 
    11   exception Floating_point of string
    12   val approx_dec_by_bin: int -> Float.float -> Float.float * Float.float
    13   val approx_decstr_by_bin: int -> string -> Float.float * Float.float
    14 
    15   val mk_float: Float.float -> term
    16   val dest_float: term -> Float.float
    17 
    18   val approx_float: int -> (Float.float * Float.float -> Float.float * Float.float)
    19     -> string -> term * term
    20 end;
    21 
    22 structure FloatArith : FLOAT_ARITH =
    23 struct
    24 
    25 exception Destruct_floatstr of string;
    26 
    27 fun destruct_floatstr isDigit isExp number =
    28   let
    29     val numlist = filter (not o Char.isSpace) (String.explode number)
    30 
    31     fun countsigns ((#"+")::cs) = countsigns cs
    32       | countsigns ((#"-")::cs) =
    33       let
    34         val (positive, rest) = countsigns cs
    35       in
    36         (not positive, rest)
    37       end
    38       | countsigns cs = (true, cs)
    39 
    40     fun readdigits [] = ([], [])
    41       | readdigits (q as c::cs) =
    42       if (isDigit c) then
    43         let
    44           val (digits, rest) = readdigits cs
    45         in
    46           (c::digits, rest)
    47         end
    48       else
    49         ([], q)
    50 
    51     fun readfromexp_helper cs =
    52       let
    53         val (positive, rest) = countsigns cs
    54         val (digits, rest') = readdigits rest
    55       in
    56         case rest' of
    57           [] => (positive, digits)
    58           | _ => raise (Destruct_floatstr number)
    59       end
    60 
    61     fun readfromexp [] = (true, [])
    62       | readfromexp (c::cs) =
    63       if isExp c then
    64         readfromexp_helper cs
    65       else
    66         raise (Destruct_floatstr number)
    67 
    68     fun readfromdot [] = ([], readfromexp [])
    69       | readfromdot ((#".")::cs) =
    70       let
    71         val (digits, rest) = readdigits cs
    72         val exp = readfromexp rest
    73       in
    74         (digits, exp)
    75       end
    76       | readfromdot cs = readfromdot ((#".")::cs)
    77 
    78     val (positive, numlist) = countsigns numlist
    79     val (digits1, numlist) = readdigits numlist
    80      val (digits2, exp) = readfromdot numlist
    81   in
    82     (positive, String.implode digits1, String.implode digits2, fst exp, String.implode (snd exp))
    83   end
    84 
    85 exception Floating_point of string;
    86 
    87 val ln2_10 = Math.ln 10.0 / Math.ln 2.0;
    88 fun exp5 x = Integer.pow x 5;
    89 fun exp10 x = Integer.pow x 10;
    90 
    91 fun find_most_significant q r =
    92   let
    93     fun int2real i =
    94       case (Real.fromString o string_of_int) i of
    95         SOME r => r
    96         | NONE => raise (Floating_point "int2real")
    97     fun subtract (q, r) (q', r') =
    98       if r <= r' then
    99         (q - q' * exp10 (r' - r), r)
   100       else
   101         (q * exp10 (r - r') - q', r')
   102     fun bin2dec d =
   103       if 0 <= d then
   104         (Integer.square d, 0)
   105       else
   106         (exp5 (~ d), d)
   107 
   108     val L = Real.floor (int2real (IntInf.log2 q) + int2real r * ln2_10)
   109     val L1 = L + 1
   110 
   111     val (q1, r1) = subtract (q, r) (bin2dec L1) 
   112   in
   113     if 0 <= q1 then
   114       let
   115         val (q2, r2) = subtract (q, r) (bin2dec (L1 + 1))
   116       in
   117         if 0 <= q2 then
   118           raise (Floating_point "find_most_significant")
   119         else
   120           (L1, (q1, r1))
   121       end
   122     else
   123       let
   124         val (q0, r0) = subtract (q, r) (bin2dec L)
   125       in
   126         if 0 <= q0 then
   127           (L, (q0, r0))
   128         else
   129           raise (Floating_point "find_most_significant")
   130       end
   131   end
   132 
   133 fun approx_dec_by_bin n (q,r) =
   134   let
   135     fun addseq acc d' [] = acc
   136       | addseq acc d' (d::ds) = addseq (acc + Integer.square (d - d')) d' ds
   137 
   138     fun seq2bin [] = (0, 0)
   139       | seq2bin (d::ds) = (addseq 0 d ds + 1, d)
   140 
   141     fun approx d_seq d0 precision (q,r) =
   142       if q = 0 then
   143         let val x = seq2bin d_seq in
   144           (x, x)
   145         end
   146       else
   147         let
   148           val (d, (q', r')) = find_most_significant q r
   149         in
   150           if precision < d0 - d then
   151             let
   152               val d' = d0 - precision
   153               val x1 = seq2bin (d_seq)
   154               val x2 = (fst x1 * Integer.square (snd x1 - d') + 1,  d') (* = seq2bin (d'::d_seq) *)
   155             in
   156               (x1, x2)
   157             end
   158           else
   159             approx (d::d_seq) d0 precision (q', r')
   160         end
   161 
   162     fun approx_start precision (q, r) =
   163       if q = 0 then
   164         ((0, 0), (0, 0))
   165       else
   166         let
   167           val (d, (q', r')) = find_most_significant q r
   168         in
   169           if precision <= 0 then
   170             let
   171               val x1 = seq2bin [d]
   172             in
   173               if q' = 0 then
   174                 (x1, x1)
   175               else
   176                 (x1, seq2bin [d + 1])
   177             end
   178           else
   179             approx [d] d precision (q', r')
   180         end
   181   in
   182     if 0 <= q then
   183       approx_start n (q,r)
   184     else
   185       let
   186         val ((a1,b1), (a2, b2)) = approx_start n (~ q, r)
   187       in
   188         ((~ a2, b2), (~ a1, b1))
   189       end
   190   end
   191 
   192 fun approx_decstr_by_bin n decstr =
   193   let
   194     fun str2int s = the_default 0 (Int.fromString s)
   195     fun signint p x = if p then x else ~ x
   196 
   197     val (p, d1, d2, ep, e) = destruct_floatstr Char.isDigit (fn e => e = #"e" orelse e = #"E") decstr
   198     val s = size d2
   199 
   200     val q = signint p (str2int d1 * exp10 s + str2int d2)
   201     val r = signint ep (str2int e) - s
   202   in
   203     approx_dec_by_bin n (q,r)
   204   end
   205 
   206 fun mk_float (a, b) = @{term "float"} $
   207   HOLogic.mk_prod (pairself (HOLogic.mk_number HOLogic.intT) (a, b));
   208 
   209 fun dest_float (Const ("Float.float", _) $ (Const ("Pair", _) $ a $ b)) =
   210       pairself (snd o HOLogic.dest_number) (a, b)
   211   | dest_float t = ((snd o HOLogic.dest_number) t, 0);
   212 
   213 fun approx_float prec f value =
   214   let
   215     val interval = approx_decstr_by_bin prec value
   216     val (flower, fupper) = f interval
   217   in
   218     (mk_float flower, mk_float fupper)
   219   end;
   220 
   221 end;