Rewrite Unicode section

Author: Maia-Everett

src/characters/unicode.md

@@ -1,21 +1,37 @@
 # Unicode
 
-Most letters and symbols that are common in the English speaking world fit into
-a single `char`, so pretending that a `char` is always "a single
-letter or symbol" is generally a good enough mental model.
+At the lowest level, computers only work with numbers and have no comprehension of text. A Java `char` is just a number between 0 and 65535. To work with text, we need to agree how to represent strings as numbers.
 
-Where this falls apart is with things like emoji (👨‍🍳) which are generally considered to be one symbol, but
-cannot be represented in a single `char`.
+The good news is that there is an international standard for encoding strings as numbers, called [Unicode](https://en.wikipedia.org/wiki/Unicode), and everyone, including Java, has agreed to follow it.
 
-```java,no_run
-char chef = '👨‍🍳';
+The bad news is that Unicode is complicated, because human writing is complicated.
+
+Unicode represents text as sequences of numbers called *code points*. As long as you only work with European languages, including English, you can pretend that a code point is just a Java `char`. For example, the letter *D* is assigned to code point 68, so the following are equivalent:
+
+```java
+~void main() {
+char letterD = 'D';
+char alsoLetterD = 68;
+IO.println(letterD == alsoLetterD); // true
+~}
+```
+
+However, not all Unicode code points fit into a `char`. A `char` can only have values between 0 and 65535, but Unicode code points can have values between 0 and 1,114,111. For example, the emoji 👨‍🍳 (code point 128104) cannot be represented by a single `char`:
+
+```java,no_run,does_not_compile
+char chef = '👨‍🍳'; // Does not compile
 ```
 
-`char`s are actually "utf-16 code units". Many symbols require multiple "code units" to represent.
+Code points that cannot fit into a single `char` are represented as two `char`s, according to the rules of the Unicode encoding called [UTF-16](https://en.wikipedia.org/wiki/UTF-16), which Java uses. UTF-16 specifies the rules for encoding Unicode text as sequences of integers between 0 and 65535 (i.e. `char` values).
+
+Another wrinkle is that a code point does not necessarily correspond to a single character shown on screen. For example, the flag of the European Union 🇪🇺 looks like a single character on screen, but is actually composed of two code points: 🇪 and 🇺 (code points 127466 and 127482 respectively).
+
+Because of these gotchas, you should only work with individual `char` values if you know what you're doing. Most of the time, you will work with whole strings, which are the topic of the [next section](/strings.html).
+
+To get a basic understanding of character encodings, you can read Joel Spolsky's *[The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)](https://www.joelonsoftware.com/2003/10/08/the-absolute-minimum-every-software-developer-absolutely-positively-must-know-about-unicode-and-character-sets-no-excuses/)*.
 
-For a full explanation, refer to this old Computerphile video.
+UTF-16, used by Java, is not the only way of encoding Unicode as a stream of bytes. Another, far more popular encoding is [UTF-8](https://en.wikipedia.org/wiki/UTF-8), which is used by most web pages. Java uses UTF-16 to represent strings internally, but uses UTF-8 for input/output by default.
 
-It describes "utf-8", which is 8 bits per "code unit." Java's `char`
-uses 16 bits, but that is the only difference.
+For a full explanation of UTF-8, refer to this old Computerphile video:
 
 <iframe width="560" height="315" src="https://www.youtube.com/embed/MijmeoH9LT4" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>