--  Copyright © 2026 Lingenic LLC. All rights reserved.
--  Licensed under the Lingenic Source-Available License v2.3.
--  Production use requires a separate license from Licensor.
--  See LICENSE.md and COPYRIGHT in the project root.
--

-------------------------------------------------------------------------------
--  Lingenic-Text
--  Formally Verified Unicode Text Processing Library
--
--  UTF-8 implementation body.
--
--  The structure mirrors the RFC 3629 §4 ABNF exactly:
--    - Lead byte determines sequence length
--    - Subsequent byte range checks match the ABNF productions
--    - Decode/encode arithmetic matches the ghost spec formulas
-------------------------------------------------------------------------------

package body Lingenic_Text.UTF8
   with SPARK_Mode
is

   procedure Decode
     (Source : Byte_Array;
      Pos    : Positive;
      CP     : out Codepoint;
      Length : out Positive;
      Valid  : out Boolean)
   is
      B0 : constant Byte := Source (Pos);
   begin
      if B0 <= 16#7F# then
         --  UTF8-1: %x00-7F
         CP     := B0;
         Length := 1;
         Valid  := True;

      elsif B0 <= 16#C1# then
         --  80..BF: continuation (not a valid lead)
         --  C0..C1: overlong 2-byte for ASCII range
         CP     := 0;
         Length := 1;
         Valid  := False;

      elsif B0 <= 16#DF# then
         --  UTF8-2: %xC2-DF UTF8-tail
         if Source'Last - Pos >= 1
           and then Source (Pos + 1) in 16#80# .. 16#BF#
         then
            CP     := (B0 - 16#C0#) * 64
                      + (Source (Pos + 1) - 16#80#);
            Length := 2;
            Valid  := True;
         else
            CP     := 0;
            Length := 1;
            Valid  := False;
         end if;

      elsif B0 <= 16#EF# then
         --  UTF8-3: E0 A0-BF tail / E1-EC tail tail /
         --          ED 80-9F tail / EE-EF tail tail
         if Source'Last - Pos >= 2
           and then
             ((B0 = 16#E0#
               and then Source (Pos + 1) in 16#A0# .. 16#BF#)
              or else
              (B0 in 16#E1# .. 16#EC#
               and then Source (Pos + 1) in 16#80# .. 16#BF#)
              or else
              (B0 = 16#ED#
               and then Source (Pos + 1) in 16#80# .. 16#9F#)
              or else
              (B0 in 16#EE# .. 16#EF#
               and then Source (Pos + 1) in 16#80# .. 16#BF#))
           and then Source (Pos + 2) in 16#80# .. 16#BF#
         then
            CP     := (B0 - 16#E0#) * 4096
                      + (Source (Pos + 1) - 16#80#) * 64
                      + (Source (Pos + 2) - 16#80#);
            Length := 3;
            Valid  := True;
         else
            CP     := 0;
            Length := 1;
            Valid  := False;
         end if;

      elsif B0 <= 16#F4# then
         --  UTF8-4: F0 90-BF tail tail / F1-F3 tail tail tail /
         --          F4 80-8F tail tail
         if Source'Last - Pos >= 3
           and then
             ((B0 = 16#F0#
               and then Source (Pos + 1) in 16#90# .. 16#BF#)
              or else
              (B0 in 16#F1# .. 16#F3#
               and then Source (Pos + 1) in 16#80# .. 16#BF#)
              or else
              (B0 = 16#F4#
               and then Source (Pos + 1) in 16#80# .. 16#8F#))
           and then Source (Pos + 2) in 16#80# .. 16#BF#
           and then Source (Pos + 3) in 16#80# .. 16#BF#
         then
            CP     := (B0 - 16#F0#) * 262144
                      + (Source (Pos + 1) - 16#80#) * 4096
                      + (Source (Pos + 2) - 16#80#) * 64
                      + (Source (Pos + 3) - 16#80#);
            Length := 4;
            Valid  := True;
         else
            CP     := 0;
            Length := 1;
            Valid  := False;
         end if;

      else
         --  F5..FF: would encode > U+10FFFF
         CP     := 0;
         Length := 1;
         Valid  := False;
      end if;
   end Decode;

   procedure Encode
     (CP     : Codepoint;
      Target : in out Byte_Array;
      Pos    : Positive;
      Length : out Positive)
   is
   begin
      if CP <= 16#7F# then
         Target (Pos) := CP;
         Length := 1;

      elsif CP <= 16#7FF# then
         Target (Pos)     := 16#C0# + CP / 64;
         Target (Pos + 1) := 16#80# + CP mod 64;
         Length := 2;

      elsif CP <= 16#FFFF# then
         Target (Pos)     := 16#E0# + CP / 4096;
         Target (Pos + 1) := 16#80# + (CP / 64) mod 64;
         Target (Pos + 2) := 16#80# + CP mod 64;
         Length := 3;

      else
         Target (Pos)     := 16#F0# + CP / 262144;
         Target (Pos + 1) := 16#80# + (CP / 4096) mod 64;
         Target (Pos + 2) := 16#80# + (CP / 64) mod 64;
         Target (Pos + 3) := 16#80# + CP mod 64;
         Length := 4;
      end if;
   end Encode;

end Lingenic_Text.UTF8;