Add initial prototype.

3-mid/opengl/private/gid/gid-decoding_png-huffman.ads

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+-- GID.Decoding_PNG.Huffman
+---------------------------
+-- Huffman tree generation and deletion.
+-- Copy of UnZip.Decompress.Huffman
+
+private package GID.Decoding_PNG.Huffman is
+
+  -- Variants A and B.
+
+  -- A/ Simplistic huffman trees, pointerless
+
+  type Length_code_pair is record
+    length: Natural;
+    code  : Natural;
+  end record;
+
+  type Huff_descriptor is array(Natural range <>) of Length_code_pair;
+
+  nil: constant:= 0;
+  root: constant:= 1;
+
+  type Huff_node is record
+    n: Natural; -- value
+    zero, one: Natural:= nil; -- index of next node, if any
+  end record;
+
+  max_size: constant:= 800;
+
+  type Huff_node_list is array(1..max_size) of Huff_node;
+
+  type Huff_tree is record
+    last: Natural:= nil;
+    node: Huff_node_list;
+  end record;
+
+  procedure Build(t: out Huff_tree; descr: in Huff_descriptor);
+
+  -- B/ Huffman tables: several steps in the binary tree
+  -- in one jump.
+  -- Pro: probably faster
+  -- Contra: complicated, relies on pointers, large data.
+
+  type HufT_table;
+  type p_HufT_table is access HufT_table;
+
+  invalid: constant:= 99; -- invalid value for extra bits
+
+  type HufT is record
+    extra_bits : Natural:= invalid;
+    bits       : Natural;
+    n          : Natural;
+    next_table : p_HufT_table:= null;
+  end record;
+
+  type HufT_table is array( Integer range <> ) of aliased HufT;
+
+  type p_HufT is access all HufT;
+
+  -- Linked list just for destroying Huffman tables
+
+  type Table_list;
+  type p_Table_list is access Table_list;
+
+  type Table_list is record
+    table: p_HufT_table;
+    next : p_Table_list;
+  end record;
+
+  type Length_array is array(Integer range <>) of Natural;
+
+  empty : constant Length_array( 1..0 ):= ( others=> 0 );
+
+  -- Free huffman tables starting with table where t points to
+  procedure HufT_free ( tl: in out p_Table_list );
+
+  -- Build huffman table from code lengths given by array b.all
+  procedure HufT_build ( b    : Length_array;
+                         s    : Integer;
+                         d, e : Length_array;
+                         tl   :    out p_Table_list;
+                         m    : in out Integer;
+              huft_incomplete :    out Boolean);
+
+  -- Possible exceptions occuring in huft_build
+  huft_error,                    -- bad tree constructed
+  huft_out_of_memory: exception; -- not enough memory
+
+end GID.Decoding_PNG.Huffman;