MeterVault/tests/Core.Tests/Analysis/FormulaParserTests.cs

using MeterVault.Core.Analysis.Virtual;

namespace MeterVault.Core.Tests.Analysis;

/// <summary>
/// The formula grammar is user input evaluated on every read, so the parser is the safety boundary (D-26): it
/// accepts only numbers, <c>m&lt;id&gt;</c>, unary minus, <c>+ - * /</c> and parentheses, reports every rejection
/// with a position instead of throwing, and cannot be driven into a stack overflow. Ported from the old
/// ExpressionEvaluatorTests; the one deliberate change is that an unknown identifier is now an error, not 0.
/// </summary>
public sealed class FormulaParserTests
{
    private static double Evaluate(string text) =>
        Formula.Parse(text).Evaluate(id => id switch { 1 => 411, 2 => 416, _ => throw new KeyNotFoundException($"m{id}") });

    [Theory]
    [InlineData("1 + 2 * 3", 7)]
    [InlineData("(1 + 2) * 3", 9)]
    [InlineData("-5", -5)]
    [InlineData("--5", 5)]
    [InlineData("+5", 5)]
    [InlineData("2 * -3", -6)]
    [InlineData("-(1 + 2) * 3", -9)]
    [InlineData("10 / 4", 2.5)]
    [InlineData("2 - 3 - 4", -5)]         // left-associative
    [InlineData("8 / 4 / 2", 1)]          // left-associative
    [InlineData("0.5 * 4", 2)]
    [InlineData(".5 + 5.", 5.5)]
    [InlineData("m1 - m2", -5)]           // Netz Einsparung Okt 2022: Haus 411 − Netz 416
    [InlineData("m1 + m2", 827)]
    [InlineData("m1+m2", 827)]
    [InlineData("  ( m1 )  -  m2  ", -5)]
    public void Evaluates_arithmetic_over_numbers_and_meter_references(string text, double expected)
    {
        Assert.Equal(expected, Evaluate(text), 9);
    }

    [Fact]
    public void Numbers_are_invariant_decimals_whatever_the_current_culture()
    {
        var previous = Thread.CurrentThread.CurrentCulture;
        try
        {
            Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("de-DE");

            Assert.Equal(0.25, Formula.Parse("0.25").Evaluate(_ => 0), 12);
            Assert.Equal(FormulaErrorKind.UnexpectedCharacter, FormulaParser.Parse("0,25").Error!.Kind);
        }
        finally
        {
            Thread.CurrentThread.CurrentCulture = previous;
        }
    }

    [Theory]
    [InlineData("1 +", FormulaErrorKind.UnexpectedEnd, 3, null)]
    [InlineData("(1 + 2", FormulaErrorKind.MissingClosingParenthesis, 0, "(")]
    [InlineData("m1 + (m2 * (3 - 1)", FormulaErrorKind.MissingClosingParenthesis, 5, "(")]
    [InlineData("1 2", FormulaErrorKind.UnexpectedToken, 2, "2")]
    [InlineData("m1 m2", FormulaErrorKind.UnexpectedToken, 3, "m2")]
    [InlineData(")", FormulaErrorKind.UnexpectedToken, 0, ")")]
    [InlineData("(m1))", FormulaErrorKind.UnexpectedToken, 4, ")")]
    [InlineData("m1 * * m2", FormulaErrorKind.UnexpectedToken, 5, "*")]
    [InlineData("1 % 2", FormulaErrorKind.UnexpectedCharacter, 2, "%")]
    [InlineData("1.2.3", FormulaErrorKind.InvalidNumber, 0, "1.2.3")]
    [InlineData(".", FormulaErrorKind.InvalidNumber, 0, ".")]
    [InlineData("1e3", FormulaErrorKind.UnexpectedToken, 1, "e3")]
    [InlineData("2m1", FormulaErrorKind.UnexpectedToken, 1, "m1")]
    [InlineData("m99999999999", FormulaErrorKind.MeterIdOutOfRange, 0, "m99999999999")]
    public void Rejects_malformed_formulas_with_the_position_and_token(string text, FormulaErrorKind kind, int position, string? token)
    {
        var result = FormulaParser.Parse(text);

        Assert.False(result.Success);
        Assert.Equal(new FormulaError(kind, position, token), result.Error);
    }

    [Theory]
    [InlineData(null)]
    [InlineData("")]
    [InlineData("   ")]
    public void Empty_text_is_an_error_not_an_exception(string? text)
    {
        Assert.Equal(FormulaErrorKind.Empty, FormulaParser.Parse(text).Error!.Kind);
    }

    [Theory]
    [InlineData("unknown + 1", "unknown", 0)] // the old evaluator read this as 1
    [InlineData("m1 - n2", "n2", 5)]
    [InlineData("M1 + m2", "M1", 0)]
    [InlineData("m1a", "m1a", 0)]
    [InlineData("m_1", "m_1", 0)]
    [InlineData("m", "m", 0)]
    [InlineData("sum(m1)", "sum", 0)]
    [InlineData("m1 + Hausverbrauch", "Hausverbrauch", 5)]
    public void Only_m_digits_identifiers_are_meter_references_and_anything_else_is_named(string text, string identifier, int position)
    {
        Assert.Equal(new FormulaError(FormulaErrorKind.UnknownIdentifier, position, identifier), FormulaParser.Parse(text).Error);
    }

    [Fact]
    public void Nesting_64_parentheses_deep_is_accepted_and_65_is_rejected_at_the_65th()
    {
        static string Nested(int depth) => new string('(', depth) + "m1" + new string(')', depth);

        Assert.True(FormulaParser.Parse(Nested(FormulaParser.MaxDepth)).Success);

        var tooDeep = FormulaParser.Parse(Nested(FormulaParser.MaxDepth + 1));
        Assert.Equal(new FormulaError(FormulaErrorKind.TooDeep, 64, "("), tooDeep.Error);
    }

    [Fact]
    public void Two_thousand_characters_are_accepted_and_2001_rejected()
    {
        var chain = "m1" + string.Concat(Enumerable.Repeat(" + m1", 399)); // 1,997 characters
        var exactly = chain + "   ";
        Assert.Equal(FormulaParser.MaxLength, exactly.Length);

        var accepted = FormulaParser.Parse(exactly);
        Assert.True(accepted.Success);
        Assert.Equal(400 * 411, accepted.Formula.Evaluate(_ => 411), 6);

        Assert.Equal(FormulaErrorKind.TooLong, FormulaParser.Parse(exactly + " ").Error!.Kind);
    }

    [Fact]
    public void A_hundred_thousand_open_parentheses_are_rejected_without_overflowing_the_stack()
    {
        Assert.Equal(FormulaErrorKind.TooLong, FormulaParser.Parse(new string('(', 100_000)).Error!.Kind);
        Assert.Equal(FormulaErrorKind.TooDeep, FormulaParser.Parse(new string('(', 1_999)).Error!.Kind);
    }

    [Fact]
    public void Long_operator_chains_and_sign_runs_parse_and_evaluate_without_recursion()
    {
        var signs = FormulaParser.Parse(new string('-', 1_999) + "1");
        Assert.True(signs.Success);
        Assert.Equal(-1, signs.Formula.Evaluate(_ => 0));

        var product = FormulaParser.Parse("1" + string.Concat(Enumerable.Repeat(" * 1", 499)));
        Assert.True(product.Success);
        Assert.Equal(1, product.Formula.Evaluate(_ => 0));
        Assert.Equal(product.Formula, Formula.Parse(product.Formula.ToString()));
    }

    [Fact]
    public void Scanning_finds_meter_tokens_even_in_text_that_does_not_parse()
    {
        Assert.Equal([2, 5, 12], FormulaParser.ScanMeterIds("m12 + (m5 * m2 +"));
        Assert.Empty(FormulaParser.ScanMeterIds("1.5 + x2"));
        Assert.Empty(FormulaParser.ScanMeterIds(null));
    }

    [Fact]
    public void Rewriting_ids_in_text_keeps_the_users_spacing_and_even_a_syntax_error()
    {
        var map = new Dictionary<int, int> { [1] = 41, [2] = 42 };

        Assert.Equal("m41  -( m42 )", FormulaParser.RewriteMeterIds("m1  -( m2 )", id => map[id]));
        Assert.Equal("m41 + ", FormulaParser.RewriteMeterIds("m1 + ", id => map[id]));
        Assert.Equal("0.1m99 + xm1", FormulaParser.RewriteMeterIds("0.1m2 + xm1", _ => 99)); // "xm1" names no meter
    }
}