Fixed "dx" in integrals, and created calculus.rs

Previously, eg. "dx" would not be parsed as "dx" after a function, since
the parser did not keep track of whether or not it was currently inside
an integral or not, properly. This commit fixes that, and also makes it
possible to use any variable after the "d", eg. "dy". The integration
function was also put in its own file: calculus.rs.

kalk/src/calculus.rs

@@ -0,0 +1,78 @@
+use crate::ast::Expr;
+use crate::ast::Stmt;
+use crate::interpreter;
+use crate::kalk_num::KalkNum;
+use crate::lexer::TokenKind;
+use crate::parser::CalcError;
+
+pub fn integrate(
+    context: &mut interpreter::Context,
+    expressions: &[Expr],
+) -> Result<KalkNum, CalcError> {
+    let mut result = KalkNum::default();
+    let mut integration_variable: Option<&str> = None;
+
+    // integral(a, b, expr dx)
+    if let Expr::Binary(_, TokenKind::Star, right) = &expressions[2] {
+        if let Expr::Var(right_name) = &**right {
+            if right_name.starts_with("d") {
+                // Take the value, but remove the d, so that only eg. x is left from dx
+                integration_variable = Some(&right_name[1..]);
+            }
+        }
+    }
+
+    if integration_variable.is_none() {
+        return Err(CalcError::ExpectedDx);
+    }
+
+    // delta_x/2[f(a) + 2f(x_1) + 2f(x_2) + ...2f(x_n) + f(b)]
+    // where delta_x = (b - a) / n
+    // and x_n = a + i * delta_x
+
+    // f(a)
+    context.symbol_table.set(Stmt::VarDecl(
+        integration_variable.unwrap().into(),
+        Box::new(expressions[0].clone()),
+    ));
+
+    // "dx" is still in the expression. Set dx = 1, so that it doesn't affect the expression value.
+    context.symbol_table.set(Stmt::VarDecl(
+        format!("d{}", integration_variable.unwrap()),
+        Box::new(Expr::Literal(1f64)),
+    ));
+
+    result.value += interpreter::eval_expr(context, &expressions[2], "")?.value;
+
+    // 2f(x_n)
+    // where x_n = a + i * delta_x
+    const N: i32 = 100;
+    let a = interpreter::eval_expr(context, &expressions[0], "")?
+        .value
+        .to_f64();
+    let b = interpreter::eval_expr(context, &expressions[1], "")?
+        .value
+        .to_f64();
+    let delta_x = (b - a) / N as f64;
+    for i in 1..N {
+        context.symbol_table.set(Stmt::VarDecl(
+            integration_variable.unwrap().into(),
+            Box::new(Expr::Literal(a + i as f64 * delta_x)),
+        ));
+
+        // 2f(x_n)
+        result.value += 2 * interpreter::eval_expr(context, &expressions[2], "")?.value;
+    }
+
+    // f(b)
+    context.symbol_table.set(Stmt::VarDecl(
+        integration_variable.unwrap().into(),
+        Box::new(expressions[1].clone()),
+    ));
+    result.value += interpreter::eval_expr(context, &expressions[2], "")?.value;
+
+    // Finally, delta_x/2 for all of it
+    result.value *= delta_x / 2f64;
+
+    return Ok(result);
+}

kalk/src/interpreter.rs

@@ -1,4 +1,5 @@
 use crate::ast::{Expr, Stmt};
+use crate::calculus;
 use crate::kalk_num::KalkNum;
 use crate::lexer::TokenKind;
 use crate::parser::CalcError;
@@ -7,7 +8,7 @@ use crate::prelude;
 use crate::symbol_table::SymbolTable;
 
 pub struct Context<'a> {
-    symbol_table: &'a mut SymbolTable,
+    pub symbol_table: &'a mut SymbolTable,
     angle_unit: String,
     #[cfg(feature = "rug")]
     precision: u32,
@@ -92,7 +93,11 @@ fn eval_expr_stmt(context: &mut Context, expr: &Expr) -> Result<KalkNum, CalcErr
     eval_expr(context, &expr, "")
 }
 
-fn eval_expr(context: &mut Context, expr: &Expr, unit: &str) -> Result<KalkNum, CalcError> {
+pub(crate) fn eval_expr(
+    context: &mut Context,
+    expr: &Expr,
+    unit: &str,
+) -> Result<KalkNum, CalcError> {
     #[cfg(not(target_arch = "wasm32"))]
     if let (Ok(elapsed), Some(timeout)) = (context.start_time.elapsed(), context.timeout) {
         if elapsed.as_millis() >= timeout {
@@ -312,68 +317,7 @@ fn eval_fn_call_expr(
                 ));
             }
 
-            let mut result = KalkNum::default();
-            let mut integration_variable: Option<&str> = None;
-
-            // integral(a, b, expr dx)
-            if let Expr::Binary(_, TokenKind::Star, right) = &expressions[2] {
-                if let Expr::Var(right_name) = &**right {
-                    if right_name.starts_with("d") {
-                        // Take the value, but remove the d, so that only eg. x is left from dx
-                        integration_variable = Some(&right_name[1..]);
-                    }
-                }
-            }
-
-            if integration_variable.is_none() {
-                unimplemented!(); // TODO: Error message
-            }
-
-            // delta_x/2[f(a) + 2f(x_1) + 2f(x_2) + ...2f(x_n) + f(b)]
-            // where delta_x = (b - a) / n
-            // and x_n = a + i * delta_x
-
-            // f(a)
-            context.symbol_table.set(Stmt::VarDecl(
-                integration_variable.unwrap().into(),
-                Box::new(expressions[0].clone()),
-            ));
-
-            // "dx" is still in the expression. Set dx = 1, so that it doesn't affect the expression value.
-            context.symbol_table.set(Stmt::VarDecl(
-                String::from("dx"),
-                Box::new(Expr::Literal(1f64)),
-            ));
-
-            result.value += eval_expr(context, &expressions[2], "")?.value;
-
-            // 2f(x_n)
-            // where x_n = a + i * delta_x
-            const N: i32 = 100;
-            let a = eval_expr(context, &expressions[0], "")?.value.to_f64();
-            let b = eval_expr(context, &expressions[1], "")?.value.to_f64();
-            let delta_x = (b - a) / N as f64;
-            for i in 1..N {
-                context.symbol_table.set(Stmt::VarDecl(
-                    integration_variable.unwrap().into(),
-                    Box::new(Expr::Literal(a + i as f64 * delta_x)),
-                ));
-
-                // 2f(x_n)
-                result.value += 2 * eval_expr(context, &expressions[2], "")?.value;
-            }
-
-            // f(b)
-            context.symbol_table.set(Stmt::VarDecl(
-                integration_variable.unwrap().into(),
-                Box::new(expressions[1].clone()),
-            ));
-            result.value += eval_expr(context, &expressions[2], "")?.value;
-
-            // Finally, delta_x/2 for all of it
-            result.value *= delta_x / 2f64;
-
-            return Ok(result);
+            return calculus::integrate(context, expressions);
         }
         _ => (),
     }

kalk/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod ast;
+mod calculus;
 mod interpreter;
 mod inverter;
 pub mod kalk_num;

kalk/src/parser.rs

@@ -92,6 +92,7 @@ impl Default for Context {
 /// Error that occured during parsing or evaluation.
 #[derive(Debug, Clone, PartialEq)]
 pub enum CalcError {
+    ExpectedDx,
     IncorrectAmountOfArguments(usize, String, usize),
     InvalidNumberLiteral(String),
     InvalidOperator,
@@ -110,6 +111,7 @@ pub enum CalcError {
 impl ToString for CalcError {
     fn to_string(&self) -> String {
         match self {
+            CalcError::ExpectedDx => format!("Expected eg. dx, to specify for which variable the operation is being done to. Example with integration: ∫(0, 1, x dx)."),
             CalcError::IncorrectAmountOfArguments(expected, func, got) => format!(
                 "Expected {} arguments for function {}, but got {}.",
                 expected, func, got
@@ -493,7 +495,8 @@ fn parse_identifier(context: &mut Context) -> Result<Expr, CalcError> {
     if !parse_as_var_instead && match_token(context, TokenKind::OpenParenthesis) {
         advance(context);
 
-        if identifier.value == "integrate" || identifier.value == "∫" {
+        let is_integral = identifier.value == "integrate" || identifier.value == "∫";
+        if is_integral {
             context.is_in_integral = true;
         }
 
@@ -506,7 +509,10 @@ fn parse_identifier(context: &mut Context) -> Result<Expr, CalcError> {
         }
 
         consume(context, TokenKind::ClosedParenthesis)?;
-        context.is_in_integral = false;
+
+        if is_integral {
+            context.is_in_integral = false;
+        }
 
         return Ok(Expr::FnCall(identifier.value, parameters));
     }