String Functions in VHDL

Our example demonstrates various string operations in VHDL. Note that VHDL doesn’t have a built-in string manipulation library like some higher-level languages, so we’ll define our own functions to mimic similar functionality.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;

entity string_functions is
end string_functions;

architecture Behavioral of string_functions is
    -- Function to check if a string contains a substring
    function contains(str : string; substr : string) return boolean is
    begin
        for i in 1 to str'length - substr'length + 1 loop
            if str(i to i + substr'length - 1) = substr then
                return true;
            end if;
        end loop;
        return false;
    end function;

    -- Function to count occurrences of a character in a string
    function count(str : string; char : character) return integer is
        variable cnt : integer := 0;
    begin
        for i in str'range loop
            if str(i) = char then
                cnt := cnt + 1;
            end if;
        end loop;
        return cnt;
    end function;

    -- Function to check if a string starts with a prefix
    function has_prefix(str : string; prefix : string) return boolean is
    begin
        return str(str'left to str'left + prefix'length - 1) = prefix;
    end function;

    -- Function to check if a string ends with a suffix
    function has_suffix(str : string; suffix : string) return boolean is
    begin
        return str(str'right - suffix'length + 1 to str'right) = suffix;
    end function;

    -- Function to find the index of a substring
    function index_of(str : string; substr : string) return integer is
    begin
        for i in 1 to str'length - substr'length + 1 loop
            if str(i to i + substr'length - 1) = substr then
                return i;
            end if;
        end loop;
        return -1;
    end function;

    -- Function to join strings (limited to 2 strings for simplicity)
    function join(str1 : string; str2 : string; separator : string) return string is
    begin
        return str1 & separator & str2;
    end function;

    -- Function to repeat a string
    function repeat(str : string; n : integer) return string is
        variable result : string(1 to str'length * n);
    begin
        for i in 1 to n loop
            result((i-1)*str'length + 1 to i*str'length) := str;
        end loop;
        return result;
    end function;

    -- Function to convert string to lowercase
    function to_lower(str : string) return string is
        variable result : string(str'range);
    begin
        for i in str'range loop
            if str(i) >= 'A' and str(i) <= 'Z' then
                result(i) := character'val(character'pos(str(i)) + 32);
            else
                result(i) := str(i);
            end if;
        end loop;
        return result;
    end function;

    -- Function to convert string to uppercase
    function to_upper(str : string) return string is
        variable result : string(str'range);
    begin
        for i in str'range loop
            if str(i) >= 'a' and str(i) <= 'z' then
                result(i) := character'val(character'pos(str(i)) - 32);
            else
                result(i) := str(i);
            end if;
        end loop;
        return result;
    end function;

begin
    process
    begin
        report "Contains:   " & boolean'image(contains("test", "es"));
        report "Count:      " & integer'image(count("test", 't'));
        report "HasPrefix:  " & boolean'image(has_prefix("test", "te"));
        report "HasSuffix:  " & boolean'image(has_suffix("test", "st"));
        report "Index:      " & integer'image(index_of("test", "e"));
        report "Join:       " & join("a", "b", "-");
        report "Repeat:     " & repeat("a", 5);
        report "ToLower:    " & to_lower("TEST");
        report "ToUpper:    " & to_upper("test");
        wait;
    end process;
end Behavioral;

This VHDL code defines a set of string manipulation functions similar to those available in higher-level languages. We’ve implemented functions for checking if a string contains a substring, counting occurrences of a character, checking for prefixes and suffixes, finding the index of a substring, joining strings, repeating strings, and converting to lower and uppercase.

To use these functions, you would typically include them in a package and then use that package in your VHDL designs. The process at the end demonstrates how to use these functions and report the results.

Note that VHDL’s string handling is more limited compared to higher-level languages, and these functions are basic implementations. In real-world VHDL applications, string manipulation is less common as VHDL is primarily used for hardware description rather than text processing.

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String Functions in VHDL