Add support for conditions and loops to AbstractCodeWriter

[mtdowling]

AbstractCodeWriter now supports Mustache-like behavior by allowing conditions and loops to be evaluated in templates. Note that this does disallow using '/', '?', '^', and '#' as formatters.

Conditions are defined using ${?foo} where "foo" is the writer named parameter to evaluate. ? is used to check if a value is truthy.

The following template with a "foo" value set to "Noid":

${?foo}
Hello, ${foo:L}!
${/foo}

Evaluates to:

Hello, Noid!

^ can be used to check if a value is falsey. The following template with no set "baz" value:

${?baz}
  Yes baz
${/baz}
${^baz}
  No baz
${/baz}

Evaluates to:

  No baz

A variable is falsey if it is Boolean false, an empty Iterable, an empty Optional, an empty Map, or an empty String. A variable is truthy if it is not falsey.

If a conditional expression or loop expression is the non-whitespace text on a line and is immediately followed by a \r or \n, then the entire line is omitted from the output.

Loops can be created using ${#foo}. Each value contained in the referenced named parameter is sent through the contained template, one after the other.

The following template with a "foo" value of ["a", "b"]:

${#foo}
- ${currentKey:L}: ${currentValue:L}
${/foo}

Evaluates to:

- 0: a
- 1: b

Each iteration of the loop creates a new state in the writer that sets a named parameter named key that contains the current index of the loop or the current key of a map entry; value containing the current value of the iterable or current value of a map entry; key.first is set to true when the first value is being evaluated, and key.last is set to true when the last value is being evaluated.

SimpleCodeWriter writer = new SimpleCodeWriter()
        .trimTrailingSpaces(false)
        .insertTrailingNewline(false);
writer.putContext("foo", Arrays.asList("a", "b", "c"));
writer.write("""
        ${#foo}
        ${?key.first}
        [
        ${/key.first}
            ${value:L}${^key.last},${/key.last}
        ${?key.last}
        ]
        ${/key.last}
        ${/foo}""");

assertThat(writer.toString(), equalTo("""
        [
            a,
            b,
            c
        ]
        """);

A custom variable name can be used in loops. For example:

${#foo as key1, value1}
    - ${key1:L}: ${value1:L} (first: ${key1.first:L}, last: ${key.last:L}
${/foo}

Whitespace that comes before an expression removed by putting ~ at the beginning of an expression.

Assuming that the first positional argument is "hi":

Greeting:
    ${~L}

Expands to:

Greeting:hi

Whitespace that comes after an expression can be removed by adding ~ to the end of the expression:

${L~}

.    

Expands to:

hi.

Leading whitespace cannot be removed when using inline block alignment ('|'). The following is invalid:

    ${~C|}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[adamthom-amzn]

I wonder if you could trim down some of the bloat from currentIsFirst and currentKey, etc. Maybe something overly complicated and bad like

SimpleCodeWriter writer = new SimpleCodeWriter()
        .trimTrailingSpaces(false)
        .insertTrailingNewline(false);
writer.putContext("foo", Arrays.asList("a", "b", "c"));
writer.write("""
        ${#foo|k,v}
        ${?kIsFirst}
        [
        ${/kIsFirst}
            ${v:L}${^kIsLast},${/kIsLast}
        ${?kIsLast}
        ]
        ${/kIsLast}
        ${/foo}""");

assertThat(writer.toString(), equalTo("""
        [
            a,
            b,
            c
        ]
        """);

[mtdowling]

I pushed an update that allows loops to define a custom key and value variable name. It now defaults to key, key.first, key.last, and value.

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