How to address dynamic array data for multiple arrays?

Question

I have the code below -

pragma solidity ^0.4.18;

contract CA3 {

struct Student {
        uint16 age;
    uint16 income;
    uint16 score;
    uint16 attendance;
}

struct Employee {
    uint16 age;
    uint16 income;
    uint16 timeWorked;
}
Student[] public students;
Employee[] public employees;
function setStudent (uint16 _age, uint16 _income, uint16 _score, uint16 _attendan
ce) public {
   Student memory tempStudent = Student({age: _age, income: _income, score: _scor
e, attendance: _attendance});
        students.push(tempStudent);
//students.push(Student({age: _age, income: _income, score: _score, attendance: _
attendance}));


}
function setEmployee(uint16 _age, uint16 _income, uint16 _timeWorked) public {
Employee memory tempEmployee = Employee({age: _age, income: _income, timeWorked: 
_timeWorked});
employees.push(tempEmployee);
}

}

I tested earlier the code without Employee and the following did show the storage for students -

var n = 2;
var startSlot = web3.toBigNumber(
    web3.sha3(web3.padLeft("0", 64), { encoding: 'hex' }));
var slot = "0x" + startSlot.add(n).toString(16);

web3.eth.getStorageAt(con.address, slot, function (err, result) {
  console.log(result);
});

But when I test with Employees I get the following -

> con.students(0)
[1, 2, 3, 4]
> con.employees(0)
[11, 12, 13]

> eth.getStorageAt(con.address,0) "0x0000000000000000000000000000000000000000000000000000000000000002"
> eth.getStorageAt(con.address,1) "0x0000000000000000000000000000000000000000000000000000000000000001"

This correctly finds the number of entries in the arrays.

> var startSlot = web3.toBigNumber(
...     web3.sha3(web3.padLeft("0", 64), { encoding: 'hex' }));
undefined
> startSlot
1.8569430475105882587588266137607568536673111973893317399460219858819262702947e+76

> var slot = "0x" + startSlot.add(n).toString(16);
undefined
> slot
"0x290decd9548b62a8d60345a988386fc84ba6bc95484008f6362f93160ef3e564"

This works for students -

> n=0
0
> var slot = "0x" + startSlot.add(n).toString(16);
undefined
> eth.getStorageAt(con.address,slot)
"0x0000000000000000000000000000000000000000000000000004000300020001"

But how is Employee found? I assume we need to code a displacement somehow for the startslot.

Answer 1

The answer is to use +1 on the key as follows -

> var key = web3.sha3("0x0000000000000000000000000000000000000000000000000000000000000001", {encoding:"hex"});
undefined
> key
"0xb10e2d527612073b26eecdfd717e6a320cf44b4afac2b0732d9fcbe2b7fa0cf6"


> var bn = web3.toBigNumber(key);
undefined
> bn
8.0084422859880547211683076133703299733277748156566366325829078699459944778998e+76



> eth.getStorageAt(con.address, bn)
"0x0000000000000000000000000000000000000000000000000000000d000c000b"

> con.students(0)
[1, 2, 3, 4]
> con.employees(0)
[11, 12, 13]

I do not know why this works. But it seems that the 0 entry in key is the first array and then +1 is next.