kmath/kmath-ast/README.md

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# AST-based expression representation and operations (`kmath-ast`)
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This subproject implements the following features:
- Expression Language and its parser.
- MST as expression language's syntax intermediate representation.
- Type-safe builder of MST.
- Evaluating expressions by traversing MST.
## Dynamic expression code generation with OW2 ASM
`kmath-ast` JVM module supports runtime code generation to eliminate overhead of tree traversal. Code generator builds
a special implementation of `Expression<T>` with implemented `invoke` function.
For example, the following builder:
```kotlin
RealField.mstInField { symbol("x") + 2 }.compile()
```
… leads to generation of bytecode, which can be decompiled to the following Java class:
```java
package scientifik.kmath.asm.generated;
import java.util.Map;
import scientifik.kmath.expressions.Expression;
import scientifik.kmath.operations.RealField;
public final class AsmCompiledExpression_1073786867_0 implements Expression<Double> {
private final RealField algebra;
private final Object[] constants;
public AsmCompiledExpression_1073786867_0(RealField algebra, Object[] constants) {
this.algebra = algebra;
this.constants = constants;
}
public final Double invoke(Map<String, ? extends Double> arguments) {
return (Double)this.algebra.add(((Double)arguments.get("x")).doubleValue(), 2.0D);
}
}
```
### Example Usage
This API is an extension to MST and MSTExpression APIs. You may optimize both MST and MSTExpression:
```kotlin
RealField.mstInField { symbol("x") + 2 }.compile()
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RealField.expression("x+2".parseMath())
```
### Known issues
- The same classes may be generated and loaded twice, so it is recommended to cache compiled expressions to avoid
class loading overhead.
- This API is not supported by non-dynamic JVM implementations (like TeaVM and GraalVM) because of using class loaders.
Contributed by [Iaroslav Postovalov](https://github.com/CommanderTvis).