2 files changed, 88 insertions, 4 deletions
diff --git a/packages/instant/src/components/amount_input.tsx b/packages/instant/src/components/amount_input.tsx
index c89fb05ad..f6f562be8 100644
--- a/packages/instant/src/components/amount_input.tsx
+++ b/packages/instant/src/components/amount_input.tsx
@@ -5,7 +5,8 @@ import * as React from 'react';
 import { ColorOption } from '../style/theme';
 import { util } from '../util/util';
 
-import { Container, Input } from './ui';
+import { ScalingInput } from './scaling_input';
+import { Container, Text } from './ui';
 
 export interface AmountInputProps {
     fontColor?: ColorOption;
@@ -22,13 +23,14 @@ export class AmountInput extends React.Component<AmountInputProps> {
         const { fontColor, fontSize, value } = this.props;
         return (
             <Container borderBottom="1px solid rgba(255,255,255,0.3)" display="inline-block">
-                <Input
+                <ScalingInput
+                    startWidthCh={3.5}
+                    endWidthCh={6}
+                    startFontSizePx={45}
                     fontColor={fontColor}
-                    fontSize={fontSize}
                     onChange={this._handleChange}
                     value={!_.isUndefined(value) ? value.toString() : ''}
                     placeholder="0.00"
-                    width="2.2em"
                 />
             </Container>
         );
diff --git a/packages/instant/src/components/scaling_input.tsx b/packages/instant/src/components/scaling_input.tsx
new file mode 100644
index 000000000..ea0a925d2
--- /dev/null
+++ b/packages/instant/src/components/scaling_input.tsx
@@ -0,0 +1,82 @@
+import { BigNumber } from '@0x/utils';
+import * as _ from 'lodash';
+import * as React from 'react';
+import * as ReactDOM from 'react-dom';
+
+import { ColorOption } from '../style/theme';
+import { util } from '../util/util';
+
+import { Container, Input } from './ui';
+
+export enum ScalingInputPhase {
+    Start,
+    Scaling,
+    End,
+}
+
+export interface ScalingInputProps {
+    startWidthCh: number;
+    endWidthCh: number;
+    startFontSizePx: number;
+    value?: string;
+    onChange?: (event: React.ChangeEvent<HTMLInputElement>) => void;
+    fontColor?: ColorOption;
+    placeholder?: string;
+}
+// Magic value obtained via trial-and-error
+const scalingRateToMaintainSameWidth = 0.1;
+export class ScalingInput extends React.Component<ScalingInputProps> {
+    public static defaultProps = {
+        onChange: util.boundNoop,
+    };
+    public render(): React.ReactNode {
+        const { fontColor, onChange, placeholder, value } = this.props;
+        const phase = this._getPhase();
+        return (
+            <Input
+                fontColor={fontColor}
+                onChange={onChange}
+                value={value}
+                placeholder={placeholder}
+                fontSize={this._calculateFontSize(phase)}
+                width={this._calculateWidth(phase)}
+            />
+        );
+    }
+    private readonly _calculateFontSize = (phase: ScalingInputPhase): string => {
+        const { value, endWidthCh, startFontSizePx } = this.props;
+        if (_.isUndefined(value) || phase !== ScalingInputPhase.End) {
+            return `${startFontSizePx}px`;
+        }
+        const charactersOverMax = value.length - endWidthCh;
+        const pixelsToReduceFontSizeBy = charactersOverMax * 2;
+        const newFontSizePx = startFontSizePx - pixelsToReduceFontSizeBy;
+        return `${newFontSizePx}px`;
+    };
+    private readonly _calculateWidth = (phase: ScalingInputPhase): string => {
+        const { value, startWidthCh, endWidthCh } = this.props;
+        if (_.isUndefined(value)) {
+            return `${startWidthCh}ch`;
+        }
+        switch (phase) {
+            case ScalingInputPhase.Start:
+                return `${startWidthCh}ch`;
+            case ScalingInputPhase.Scaling:
+                return `${value.length}ch`;
+            case ScalingInputPhase.End:
+                return `${endWidthCh}ch`;
+            default:
+                return `${startWidthCh}ch`;
+        }
+    };
+    private readonly _getPhase = (): ScalingInputPhase => {
+        const { value, startWidthCh, endWidthCh } = this.props;
+        if (_.isUndefined(value) || value.length <= this.props.startWidthCh) {
+            return ScalingInputPhase.Start;
+        }
+        if (value.length > startWidthCh && value.length <= endWidthCh) {
+            return ScalingInputPhase.Scaling;
+        }
+        return ScalingInputPhase.End;
+    };
+}