Fixed issue with Decimal String conversion resulting in more than 16 … (

#367)

* Fixed issue with Decimal String conversion resulting in more than 16 characters.

* Added documentation and one more test case.

src/ValueRepresentation.js

@@ -533,13 +533,37 @@ class DecimalString extends AsciiStringRepresentation {
     }
 
     formatValue(value) {
-        if (value === null) {
-            return "";
-        }
+        if (value === null) return "";
 
-        const str = String(value);
+        let str = String(value);
         if (str.length > this.maxLength) {
-            return value.toExponential();
+            // Characters needed for '-' at start.
+            const sign_chars = value < 0 ? 1 : 0;
+
+            // Decide whether to use scientific notation.
+            const logval = Math.log10(Math.abs(value));
+
+            // Numbers larger than 1e14 cannot be correctly represented by truncating
+            // their string representations to 16 chars, e.g pi * 10^13 would become
+            // '314159265358979.', which may not be universally understood. This limit
+            // is 1e13 for negative numbers because of the minus sign.
+            // For negative exponents, the point of equal precision between scientific
+            // and standard notation is 1e-4 e.g. '0.00031415926535' and
+            // '3.1415926535e-04' are both 16 chars.
+            const use_scientific = logval < -4 || logval >= 14 - sign_chars;
+            if (use_scientific) {
+                const trunc_str = value.toExponential(16 - sign_chars);
+                if (trunc_str.length <= 16) return trunc_str;
+                // If string is too long, correct the length.
+                return value.toExponential(
+                    16 - (trunc_str.length - 16) - sign_chars
+                );
+            } else {
+                const trunc_str = value.toFixed(16 - sign_chars);
+                if (trunc_str.length <= 16) return trunc_str;
+                // If string is too long, correct the length.
+                return value.toFixed(16 - sign_chars - (trunc_str.length - 16));
+            }
         }
         return str;
     }

test/data.test.js

@@ -16,6 +16,7 @@ import datasetWithNullNumberVRs from "./mocks/null_number_vrs_dataset.json";
 import { rawTags } from "./rawTags";
 
 import { EXPLICIT_LITTLE_ENDIAN, IMPLICIT_LITTLE_ENDIAN } from "./../src/constants/dicom.js";
+import { ValueRepresentation } from "../src/ValueRepresentation";
 
 const {
     DicomMetaDictionary,
@@ -790,4 +791,27 @@ it("Tests that reading fails on a DICOM without a meta length tag", () => {
     }).toThrow(
         "Invalid DICOM file, meta length tag is malformed or not present."
     );
+});
+
+it.each([
+    [1.0, "1"],
+    [0.0, "0"],
+    [-0.0, "0"],
+    [0.123, "0.123"],
+    [-0.321, "-0.321"],
+    [0.00001, "0.00001"],
+    [3.14159265358979323846, "3.14159265358979"],
+    [-3.14159265358979323846, "-3.1415926535898"],
+    [5.3859401928763739403e-7, "5.38594019288e-7"],
+    [-5.3859401928763739403e-7, "-5.3859401929e-7"],
+    [1.2342534378125532912998323e10, "12342534378.1255"],
+    [6.40708699858767842501238e13, "64070869985876.8"],
+    [1.7976931348623157e308, "1.797693135e+308"],
+    [0.99990081787109, "0.99990081787109"],
+    ])
+    ("A converted decimal string should not exceed 16 bytes in length", (a, expected) => {
+        const decimalString = ValueRepresentation.createByTypeString("DS");
+        let value = decimalString.formatValue(a);
+        expect(value.length).toBeLessThanOrEqual(16);
+        expect(value).toBe(expected);
 });