added explicit Euler solver

src/core/solvers/SolverRK1.ts

@@ -0,0 +1,63 @@
+/** 
+ * This file is part of LoopInsighT1, an open source tool to
+ * simulate closed-loop glycemic control in type 1 diabetes.
+ * Distributed under the MIT software license.
+ * See https://lt1.org for further information.
+ */
+
+import NamedVectorUtil from '../../common/NamedVectorUtil.js'
+import { NamedVector } from '../../types/CommonTypes.js'
+import Solver, { Derivatives } from '../../types/Solver.js'
+
+const timesScalar = NamedVectorUtil.timesScalar
+const vectorSum = NamedVectorUtil.vectorSum
+const MS_PER_MIN = 60e3
+
+/**
+ * A class providing an explicit Euler numerical solver. 
+ */
+export default class SolverRK1 implements Solver {
+    /** current step size in minutes */
+    protected _timeStep = 1
+
+    reset(defaultTimeStep: number) {
+        this._timeStep = defaultTimeStep
+    }
+
+    solve(
+        derivatives: Derivatives,
+        tInit: Date,
+        xInit: NamedVector,
+        tFinal: Date,
+    ): NamedVector {
+
+        let t = tInit.valueOf()
+        let dt = this._timeStep
+        let x = xInit
+        while (t < tFinal.valueOf()) {
+            const tNext = Math.min(t + dt * MS_PER_MIN, tFinal.valueOf())
+            x = this._performTimeStep(derivatives, t, x, (tNext - t) / MS_PER_MIN)
+            t = tNext
+        }
+        return x
+    }
+
+    /**
+     * Performs single time step.
+     * 
+     * @param {function} derivatives - Function that returns dx/dt
+     * @param {Date} t - Initial time as number.
+     * @param {NamedVector} x - Initial state.
+     * @param {number} dt - Time step in minutes.
+     */
+    _performTimeStep(
+        derivatives: Derivatives,
+        t: number,
+        x: NamedVector,
+        dt: number,
+    ): NamedVector {
+        return vectorSum(x,
+            timesScalar(derivatives(new Date(t), x), dt)
+        )
+    }
+}