Add Forth solution

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+#! /usr/bin/env gforth
+
+\ Challenge 209
+\
+\ Task 1: Special Bit Characters
+\ Submitted by: Mohammad S Anwar
+\
+\ You are given an array of binary bits that ends with 0.
+\
+\ Valid sequences in the bit string are:
+\
+\ [0] -decodes-to-> "a"
+\ [1, 0] -> "b"
+\ [1, 1] -> "c"
+\
+\ Write a script to print 1 if the last character is an “a” otherwise print 0.
+\ Example 1
+\
+\ Input: @bits = (1, 0, 0)
+\ Output: 1
+\
+\ The given array bits can be decoded as 2-bits character (10) followed by
+\ 1-bit character (0).
+\
+\ Example 2
+\
+\ Input: @bits = (1, 1, 1, 0)
+\ Output: 0
+\
+\ Possible decode can be 2-bits character (11) followed by 2-bits character
+\ (10) i.e. the last character is not 1-bit character.
+
+\ line of decoded text
+CREATE output 0 C, 255 ALLOT
+0 VALUE last_output_is_a
+
+: append_output ( ch -- )
+    DUP 'a' = IF 1 TO last_output_is_a ELSE 0 TO last_output_is_a THEN
+    output COUNT + C!
+    output C@ 1+ output C!
+;
+
+: append_a 'a' append_output ;
+: append_b 'b' append_output ;
+: append_c 'c' append_output ;
+
+CREATE input 256 ALLOT
+
+: set_input { addr len -- }
+    1 input C!              \ next char pointer
+    addr input 1+ len CMOVE
+    0  input 1+ len + C!    \ end marker
+;
+
+: cur_input ( -- ch|0 )
+    input C@ input + C@
+;
+
+: next_input ( -- ch|0 )
+    cur_input 0<> IF
+        input C@ 1+ input C!
+    THEN
+    cur_input
+;
+
+: skip_input ( -- )
+    next_input DROP
+;
+
+: decode_str ( -- )
+    0 { decode_state }
+    BEGIN
+        decode_state 0= IF
+            cur_input 0= IF EXIT ELSE
+            cur_input '0' = IF append_a skip_input ELSE
+            cur_input '1' = IF 1 TO decode_state skip_input ELSE
+            EXIT
+            THEN THEN THEN
+        ELSE
+            cur_input 0= IF EXIT ELSE
+            cur_input '0' = IF append_b skip_input 0 TO decode_state ELSE
+            cur_input '1' = IF append_c skip_input 0 TO decode_state ELSE
+            EXIT
+            THEN THEN THEN
+        THEN
+    AGAIN
+;
+
+NEXT-ARG set_input
+decode_str
+last_output_is_a . CR
+BYE