using System;

namespace MathInterpreter.Core;

/// <summary>
///     Provides utility methods for scanning character streams for certain parts of a maths expression
/// </summary>
public static class ExpressionScanner
{
    /// <summary>
    ///     Scans the given expression for an operator (+, -, *, /)
    /// </summary>
    /// <param name="expression">Expression to process</param>
    /// <returns>A scan result containing the operator found and the remaining part of the expression</returns>
    /// <exception cref="OperatorException">Thrown if none or an unknown operator is found</exception>
    public static ScanResult<Operator> ScanOperator(ReadOnlySpan<char> expression)
    {
        int index = 0;
        
        while (index < expression.Length && char.IsWhiteSpace(expression[index]))
        {
            index++;
        }
    
        if (index >= expression.Length)
        {
            throw new OperatorException("Empty expression");
        }
    
        char op = expression[index];

        return op switch
               {
                   '+' => new ScanResult<Operator>(Operator.Plus, 1, expression[(index + 1)..]),
                   '-' => new ScanResult<Operator>(Operator.Minus, 1, expression[(index + 1)..]),
                   '*' => new ScanResult<Operator>(Operator.Multiply, 1, expression[(index + 1)..]),
                   '/' => new ScanResult<Operator>(Operator.Divide, 1, expression[(index + 1)..]),
                   _   => throw new OperatorException($"Unknown operator: {op}")
               };
    }

    /// <summary>
    ///     Scans the given expression for an operand which is a number which have to have an integral
    ///     part and may have a fractional part. It may also have a sign (+, -) in front of it.
    /// </summary>
    /// <param name="expression">Expression to process</param>
    /// <returns>A scan result containing the parsed number and the remaining part expression</returns>
    /// <exception cref="ExpressionFormatException">Thrown if a problem with the format of the number is found</exception>
    public static ScanResult<double> ScanOperand(ReadOnlySpan<char> expression)
    {
        int index = 0;
        
        while (index < expression.Length && char.IsWhiteSpace(expression[index]))
        {
            index++;
        }

        if (index >= expression.Length)
        {
            throw new ExpressionFormatException("Operand is missing");
        }

        if (expression[index] == '.')
        {
            throw new ExpressionFormatException("Unable to parse integral part of number");
        }
        
        if (!(char.IsDigit(expression[index]) || 
              (index < expression.Length - 1 && (expression[index] == '+' || expression[index] == '-') && char.IsDigit(expression[index + 1]))))
        {
            throw new ExpressionFormatException("Operand is missing");
        }

        try
        {
            var integralResult = ScanNumber(expression, true);
            int integralValue = integralResult.Value;
            bool isNegative = integralValue < 0;
            
            ReadOnlySpan<char> remaining = integralResult.RemainingExpression;
            index = 0;
            
            while (index < remaining.Length && char.IsWhiteSpace(remaining[index]))
            {
                index++;
            }
            
            if (index >= remaining.Length || remaining[index] != '.')
            {
                return new ScanResult<double>(integralValue, integralResult.Length, remaining);
            }
            
            ReadOnlySpan<char> fractionalPart = remaining[(index + 1)..];
            
            try
            {
                var fractionalResult = ScanNumber(fractionalPart, false);
                double fractionalValue = fractionalResult.Value / Math.Pow(10, fractionalResult.Length);
                
                double result = Math.Abs(integralValue) + fractionalValue;
                if (isNegative)
                {
                    result = -result;
                }

                return new ScanResult<double>(result,
                    integralResult.Length + 1 + fractionalResult.Length,
                    fractionalResult.RemainingExpression);
            }
            catch (NumberFormatException)
            {
                throw new ExpressionFormatException("Unable to parse fractional part of number");
            }
        }
        catch (NumberFormatException)
        {
            throw new ExpressionFormatException("Unable to parse integral part of number");
        }
    }

    /// <summary>
    ///     Scans the given expression for a whole number which may have a sign (+, -) in front of it
    /// </summary>
    /// <param name="expression">Expression to process</param>
    /// <param name="allowSign">Flag indicating if a pre-fix sign (+, -) is allowed or not</param>
    /// <returns>A scan result containing the parsed number and the remaining part expression</returns>
    /// <exception cref="NumberFormatException">Thrown if no or an invalid number is found</exception>
    public static ScanResult<int> ScanNumber(ReadOnlySpan<char> expression, bool allowSign)
    {
        int index = 0;
        int sign = 1;
        
        while (index < expression.Length && char.IsWhiteSpace(expression[index]))
        {
            index++;
        }
        
        if (index < expression.Length)
        {
            if (expression[index] == '+' || expression[index] == '-')
            {
                if (!allowSign)
                {
                    throw new NumberFormatException(expression, "Sign is not allowed here");
                }
                
                if (expression[index] == '-')
                {
                    sign = -1;
                }
                index++;
                
                while (index < expression.Length && char.IsWhiteSpace(expression[index]))
                {
                    index++;
                }
                
                if (index < expression.Length && (expression[index] == '+' || expression[index] == '-'))
                {
                    throw new NumberFormatException(expression, "Sign may occur only once");
                }
            }
        }
        
        int value = 0;
        int digitCount = 0;

        while (index < expression.Length)
        {
            char c = expression[index];

            if (char.IsDigit(c))
            {
                int digit = (int)char.GetNumericValue(c);
                value = value * 10 + digit;
                index++;
                digitCount++;
            }
            else if (char.IsWhiteSpace(c))
            {
                index++;
            }
            else
            {
                break;
            }
        }
        
        if (digitCount == 0)
        {
            throw new NumberFormatException(expression);
        }

        return new ScanResult<int>(value * sign, digitCount, expression[index..]);
    }
}