feat(function): add edge and pad to ScalarFunctions

src/main/kotlin/kestrel/engine/function/ScalarFunctions.kt

@@ -161,3 +161,91 @@ object ScalarFunctions {
         return Value.TextValue(if (start >= end) "" else source.substring(start, end))
     }
 
+    /** `LEFT(str, n)`/`RIGHT(str, n)`: the first/last `n` characters, clamped to `[0, str.length]` (a non-positive `n` is empty, an `n` past the string's length is the whole string). */
+    private fun edge(args: List<Value>, function: String, fromStart: Boolean): String {
+        val source = text(args[0], function)
+        val count = integer(args[1], function).coerceIn(0, source.length.toLong()).toInt()
+        return if (fromStart) source.substring(0, count) else source.substring(source.length - count)
+    }
+
+    /**
+     * MySQL-compatible LPAD/RPAD: the result is always exactly [target length][Value.IntegerValue]
+     * characters (never longer) - a source already at or past that length is truncated to it (from
+     * the start, regardless of which side would have been padded), and a source shorter than it is
+     * padded by cycling [padString] (default a single space) until the target length is reached.
+     */
+    private fun pad(args: List<Value>, function: String, left: Boolean): Value {
+        val source = text(args[0], function)
+        val targetLength = integer(args[1], function)
+        if (targetLength <= 0) return Value.TextValue("")
+        val length = targetLength.toInt()
+        if (length <= source.length) return Value.TextValue(source.substring(0, length))
+        val padString = if (args.size == 3) text(args[2], function) else " "
+        if (padString.isEmpty()) throw KestrelException("function $function requires a non-empty pad string")
+        val needed = length - source.length
+        val padding = buildString {
+            while (this.length < needed) append(padString)
+        }.substring(0, needed)
+        return Value.TextValue(if (left) padding + source else source + padding)
+    }
+
+    /** Two INTEGER operands with a non-negative exponent stay exact (repeated Long multiplication); anything else - a negative exponent or a REAL operand on either side - falls back to `Math.pow`. */
+    private fun power(base: Value, exponent: Value): Value {
+        if (base is Value.IntegerValue && exponent is Value.IntegerValue && exponent.value >= 0) {
+            var result = 1L
+            repeat(exponent.value.toInt()) { result *= base.value }
+            return Value.IntegerValue(result)
+        }
+        return Value.RealValue(number(base, "POWER").pow(number(exponent, "POWER")))
+    }
+
+    /** Two INTEGER operands use exact `Long` remainder; a REAL operand on either side uses floating-point remainder instead. */
+    private fun mod(dividend: Value, divisor: Value): Value {
+        if (dividend is Value.IntegerValue && divisor is Value.IntegerValue) {
+            if (divisor.value == 0L) throw KestrelException("division by zero")
+            return Value.IntegerValue(dividend.value % divisor.value)
+        }
+        val divisorNumber = number(divisor, "MOD")
+        if (divisorNumber == 0.0) throw KestrelException("division by zero")
+        return Value.RealValue(number(dividend, "MOD") % divisorNumber)
+    }
+
+    private fun round(args: List<Value>): Value {
+        val digits = if (args.size == 2) {
+            (args[1] as? Value.IntegerValue)?.value?.toInt()
+                ?: throw KestrelException("function ROUND requires an INTEGER digit count")
+        } else {
+            0
+        }
+        if (args[0] is Value.IntegerValue && digits >= 0) return args[0]
+        val decimal = args[0] as? Value.DecimalValue
+        if (decimal != null) return Value.DecimalValue(decimal.value.setScale(digits, RoundingMode.HALF_UP))
+        val value = number(args[0], "ROUND")
+        val factor = 10.0.pow(digits)
+        return Value.RealValue((value * factor).roundToLong() / factor)
+    }
+
+    private fun sign(value: Value): Int = when (value) {
+        is Value.IntegerValue -> if (value.value > 0) 1 else if (value.value < 0) -1 else 0
+        is Value.RealValue -> if (value.value > 0) 1 else if (value.value < 0) -1 else 0
+        is Value.DecimalValue -> value.value.signum()
+        else -> throw KestrelException("function SIGN requires a numeric argument but got ${value.typeName}")
+    }
+
+    /** Same digit-count/type handling as [round], but truncates toward zero (`RoundingMode.DOWN`) instead of rounding half-up. */
+    private fun trunc(args: List<Value>): Value {
+        val digits = if (args.size == 2) {
+            (args[1] as? Value.IntegerValue)?.value?.toInt()
+                ?: throw KestrelException("function TRUNC requires an INTEGER digit count")
+        } else {
+            0
+        }
+        if (args[0] is Value.IntegerValue && digits >= 0) return args[0]
+        val decimal = args[0] as? Value.DecimalValue
+        if (decimal != null) return Value.DecimalValue(decimal.value.setScale(digits, RoundingMode.DOWN))
+        val value = number(args[0], "TRUNC")
+        val factor = 10.0.pow(digits)
+        val scaled = value * factor
+        return Value.RealValue((if (scaled >= 0) floor(scaled) else ceil(scaled)) / factor)
+    }
+}