JSON templating

Stage 1

Write a cli tool using any language you like to resolve JSON templating. The usage of the tool is:

$ jsont input.json params.json

The json string template syntax is: {{variableName}}. The replace only happens in JSON string values. The left braces can be escaped by {{lb}}. The right braces do not need to be escaped. "lb" cannot be used as a variable name. Variable name can only be \w+. The application writes the result to stdout.

Stage 1 detailed requirements

1. You can use the programming language you like.
2. You can use the existing library to parse JSON.
3. You can use regex lib.
4. You cannot use existing templating library.
5. CLI usage: jsont <input.json> <params.json>, if wrong, print usage and exit code = 1.
6. If the program cannot read input.json or params.json, or they are invalid json, exit with code = 2, print error to stderr
7. params.json cannot contain nested structure, object values must all be string type. Object keys should be \w+. If not valid, exit with code = 3.
8. Var name cannot be lb in params.json, if detected, exit with code = 3.
9. The variables in params.json should not be replaced/transformed. They are all pure string literals.
10. Only string value in input.json should be replaced, object key should not be processed. String values in JSON array should be processed, too.
11. Only one pass of replacement should happen. For example: var1='value1', then {{lb}}{{lb}}var1}} should be {{var1}}; Similarly, given var1='var2', var2='value2', {{{{var1}}}} should be {{var2}}; given var1='}}', {{lb{{var1}} should be {{lb}}
12. If unknown var name is found in input.json, like {{unknown}}, then you should leave it unchanged.
13. Print the output to stdout, with indentation = 2 spaces.
14. Write test cases to verify your implementation.

Example for Stage 1

input.json:

{
  "key1": "some {{var1}}",
  "key2": {
    "{{var1}}": "{{var3}} {{var4}} {{var5}}"
  },
  "key3": [
    "{{var2}} abc",
    "escaped braces: {{lb}}{{lb}}var3}}"
  ]
}

params.json:

{
  "var1": "value1 {{var2}}",
  "var2": "value2 {{lb}}{{lb}}var3}}",
  "var3": "value3",
  "var4": "value4",
  "var5": "value5"
}

Then the output should be:

{
  "key1": "some value1 {{var2}}",
  "key2": {
    "{{var1}}": "value3 value4 value5"
  },
  "key3": [
    "value2 {{lb}}{{lb}}var3}} abc",
    "escaped braces: {{var3}}"
  ]
}

Stage 2

If you finish stage 1 and still have extra time, please consider implementing stage 2, the recursive param mode.

Add a switch -r to enable the recursive mode.

Stage 2 detailed requirements

1. The variable values now can reference other variables, recursively.
2. If there are circular references, exit with code = 4.
3. {{lb}} should be escaped to { in params.json.
4. {{unknown}} in variable values should not be processed.
5. Given var1='{{lb{{var2}}', var2='}}', var1's final value should be {{lb}}
6. Given var1='{{lb}}{{lb}}var2}}', var1's final value should be {{var2}}

Example for Stage 2

input.json:

{
  "key1": "{{var1}} {{var2} {{var3}}",
  "key2": {
    "{{var2}}": "{{var4}}"
  },
  "{{var3}}": [
    "{{var5}} {{var6}}"
  ]
}

params.json:

{
  "var1": "{{var2}}",
  "var2": "{{var3}}",
  "var3": "3",
  "var4": "4 -> {{var3}}",
  "var5": "{{varNONO}}"
}

output:

{
  "key1": "3 3 3",
  "key2": {
    "{{var2}}": "4 -> 3"
  },
  "{{var3}}": [
    "{{varNONO}} {{var6}}"
  ]
}